This is why the SPaG test (now remodelled as the GPS test) is serious, nasty and dangerous.
1. It is deliberately and intentionally too hard for many children. It is ultimately simply and only a means of grading children.
2. Because of this it penalises teachers and schools trying to teach it.
3. It's not only too hard. The way the questions in the test are framed, it's full of ambiguities and inconsistencies, particularly in the matter of categories. Example: 'determiner' groups a set of 'parts of speech' that can't be anything other than 'determiners'. 'Fronted adverbial' groups together a mix of parts of speech, phrases, clauses all of which can be other things. Example: there are serious problems with the notion of 'subordination' (i.e. 'subordinate clauses') and so with the knock-on effect of having 'subordinate conjunction' which supposedly differ from other conjunctions. Example: 'fronted adverbial' actually lumps in phrases or clauses which aren't 'adverbial'. That is, they describe the subject of the clause that follows it, as with 'With his hat on, Jack picked his nose.' 'With his hat on', is as much 'adjectival' as it is adverbial but would still count as a 'fronted adverbial'.
4. We should remember that there is (1) 'grammar' which simply means that language is words stuck together in certain ways. There is (2) 'linguistics' which is the study of language, investigating language. Then there is (3) 'SPaG grammar' which is a specific way of laying down rules about how the language should be described and written. It is 'prescriptive'. The whole trick of SPaG is to pretend that SPaG 'is' all three. It isn't. It's one specific kind of interpretation of language and teaching born out of government demands.
5. It was brought in because the government said that they wanted a means to impose 'right/wrong' tests on schools. But, spelling, punctuation and grammar questions are not 100% right/wrong. Nor should they be. Government even told linguists what specific features they wanted. Michael Gove told linguists that they had to put in questions on 'subjunctives'. The linguists said no. Gove said yes. That's how absurd and wrong SPaG is. (The reason why linguists said no, because even in their narrow, structural description of language, they acknowledge that English doesn't really have the same kind of structures that linguists usually call 'subjunctives' e.g. in French. So whatever 'WERE' 'If I WERE going to the cinema' is, they said, it isn't really a 'subjunctive'. Oh yes it is, said Gove. That's why teachers have to spend time telling children how to spot it. That's why many children won't. Because Gove said they should.
6. SPaG grammar is full of problems that many linguists argue with.
7. SPaG grammar is based on an idea that you can describe language purely on the basis of 'structure'. In fact, you can only know the structure because you know the meaning of the words but most of the terminology does not reflect that meaning has been included. Example: 'determiner' is a term to describe some of the words that come before 'nouns'. But you can only identify which ones are determiners and which ones are not, on the basis of their meaning. But the word 'determiner' doesn't tell us that. On the other hand 'possessive' at least has the virtue of telling us that the word in question tells us that we have words that tell us about how we indicate we 'possess' things. This also tells us that the terms themselves are inconsistent between them.
8. SPaG grammar is based on a false idea of what language is. That's to say, it treats language as unchanging, static and written. This is because it is based on a grammar that was devised centuries ago to describe an unchanging, static written language: Latin. They then recycled many of the terms to use to describe modern, living, changing languages and then tried to use these to impose rules. This is intellectually shoddy and there is no place for this in the modern world.
9. There are excellent alternatives to this. In 1970, teachers and experts at the School Council devised a huge resource called 'Language in Use' which enabled teachers and children to explore and investigate language - as it is actually used. There is a strong intellectual tradition which poses a different view of how knowledge about language can be taught.
10. SPaG grammar imposes a false, inaccurate and useless model of how writing can be taught. Example: teachers are being asked to define 'good' writing by asking pupils to insert 'expanded noun phrases,' 'embedded relative clauses' and 'fronted adverbials' into their writing. These are then used as criteria for getting marks when pupils use them. This is absurd and wrong. This only arises out of the government's wish to classify writing. It has nothing to do with good writing - or children! It is a perfect example of how the needs of government override the needs of the topic and the needs of children.
11. This problem is made worse by the absurdities being imposed in relation to punctuation. Across the writing of English, there is a huge variation in usage of punctuation marks. These are determined by newspapers, publishers, writers of advertising copy - not by linguists or governments. It is impossible to say which of these are right or wrong. Many of the examples given to children as right/wrong are not right/wrong. Example: the use or not use of the 'Oxford comma'.
12. It is worth remembering that no one gets SPaG grammar right all the time. My writing, your writing, our writing is littered with typos and what SPaG grammar would call wrong. Editors and copy-editors work hours and hours every day, 'correcting' the writing of people who appear in public as writers. We writers are incredibly grateful to them. If you are a powerful person, you also have people paid to check through what appears in public. Nicky Morgan, Nick Gibb, Michael Gove all make these errors and typos. Unlike children and teachers, they are not penalised for this. They have people to edit and check their writing so that it looks better than it is. Even so, errors appear in, say, DfE letters they send out to us and even in their own tests. To imply to children that it's possible to be 100% right is unpleasant, if not nasty. Meanwhile, there is a whole 'market' (!) in writing that does not obey the very same rules that government is imposing. It's the 'market' of advertising copy which children see every day of the week.
13. They have also levered in a question which is not spelling, not punctuation and not grammar. It's 'synonyms and antonyms'. This poses a false model of language - that there really are words which are 'identical' or 'opposites'. In fact, language rests precisely on the fact that there aren't. Meanwhile, the specifics of what children are asked to do on this topic, involve 'hard' words like 'meandering' and 'evade'. So it's not a test on the principle of synonymy and antonymy! If it was, it would be on words like 'good' and 'bad'. So it's a test on knowing some hard words - which is actually a test mostly on home background and culture. So 'aptitude' or 'ability to learn what's taught' is not what is actually tested here.