Veganism

Have you actually seen this in action at the school or might it be taken out of context? I find it hard to believe that teachers would be permitted to preach a personal belief to young children in this way, if it is the case, then it should be raised as a formal complaint with the head in my opinion, and I speak as a vegetarian of 30 years and now a vegan

No, you're not.
If veganism is considered a 'faith/belief' system, then no-one should be preaching it to children. They are usually taught about all faiths at school, so veganism shouldn't be pushed.

I don't mind people being vegan if they abhore the thought of eating animals. Some of their attitudes to animal exploitation can get a bit extreme, but they're entitled to their opinion. What's more, fluffy little lambs don't go to slaughter - by the time that happens they're nearly full-grown with mucky bums and not nearly so attractive.

And going vegan won't save the world because only having plants won't give a balanced ecology.

I'd complain to the school and ask for a balanced viewpoint to be given.