Labour have never had to fight very hard for my vote – but, as a primary school teacher, the thought of putting them in charge of primary education fills me with dread. With a potential snap election looming, Jeremy Corbyn announced yesterday that Labour would bring about what many might assume to be my dream scenario: an end to Sats.

Labour, he said, would raise standards by “letting teachers teach”. That would indeed be a welcome prospect. However, at the end of a child’s time at primary school, how will we know if the teaching has been any good?

In the absence of standardised tests, we would probably fall back on teachers’ own assessments to measure children’s progress and attainment. This would be a reckless leap of faith, and would increase teacher workload, while leaving our most vulnerable children at the mercy of notoriously unreliable judgments.

Sats are not currently used to assess every subject in year 6. As it stands, writing is assessed by teachers already and, for the unlucky ones, moderated externally by the local authority. If Labour’s plans bear any resemblance to the current process for assessing writing, then we should all be very worried indeed. Speak to any year 6 teacher for long enough, and you will eventually discover the heavy workload and acute misery associated with teacher assessment – turgid checklists and mountains of “evidence portfolios,” painstakingly collected over the year.

But this is not the worst of it. What makes this such a worrying prospect is the proven unreliability of teachers’ judgements. In 2017, a freedom of information request revealed that two-thirds of moderators – trained to check teacher assessments of pupils’ writing for local authorities – failed to assess pupils’ work correctly when tested. For anyone who has had to endure one of these galling lessons in subjectivity, this will come as no surprise.

For all their flaws (which I’ll get to in a moment) Sats contribute far less to teacher workload and stress than teacher assessment, which requires so much paper-shuffling. In theory, all they demand is that you turn up, teach your heart out and let the government do the rest.

In his speech at the National Education Union conference yesterday, Corbyn said he looked forward to “the rekindling of the spirit of creativity in our schools”. We should all stand united behind this worthy goal, and you won’t find many teachers disagreeing with it. However, it overlooks an important truth about creativity: no one can be creative in a subject they haven’t mastered. Creativity depends entirely on a secure foundation of knowledge, which can be won only by solid teaching backed up by a reliable system of accountability.

I’m a teacher. My family are teachers. But I just don’t understand the lazy sanctification of teachers evident in Corbyn’s speech. We all know that the vast majority work hard and raise educational standards, year after year. However, in part because of a chronic shortage of effective continuing professional development, the inconvenient truth remains that there are also thousands of terrible, terrible teachers. I have personally been in far too many bad schools to believe that we are ready to scrap standardised tests at the end of key stage 2.

Many opponents of Sats talk about the perils of high-stakes testing, and I would agree with much of this criticism. However, the stakes involved in getting children to a decent level of education before secondary school are also extremely high.

None of this is to say that Sats should be left as they are. In fact, if I had my way they would look almost unrecognisable. One huge barrier to effective standardised tests is the bloated size of our primary curriculum. As it stands, Sats ensure that children leave primary school with an education that is a mile wide but too often an inch deep.

We need properly invigilated, standardised tests that focus on the most vital knowledge and guarantee unfaltering fluency for as close to 100% of children as possible. We are still depressingly far from this ideal. With only 3.8 days per curriculum objective in maths, for example, teaching can currently feel like building a house on quicksand.

Corbyn is wrong to assume that teachers’ integrity is enough to ensure great outcomes for our children. Consider this sobering fact: in the final year that science was assessed by Sats, 88% of children reached the expected level. Since the removal of the science test, this number fell to 23% in 2016, based on the results of a national sample taking a paper. It seems that when no one is looking, other priorities take over and children’s attainment declines.

Yes, there was a part of me that was excited at the thought of no more Sats. But this was the same part of me that doesn’t want to get out of bed when the clocks go forward. If Labour cares about raising standards, then it should be reforming Sats, not scrapping them.

• The writer is a primary school teacher who blogs on education policy under the name Solomon Kingsnorth