Sir Isaac Newton - this is getting ridiculous

Of course, he could do something a litle different and go as William Gilbert. Gilbert predates Newton and is considered the father of magnetic study. A red T shirt, an old brown throw, make a paper ruff by pleating some copy paper and a cardboard top hat painted black.

They are great ideas, katek, but many people pass on old joggers until they are beyond wear, and not everyone has time to trawl charity shops for lace. And with all due respect, 'not too pricey on Amazon' means different things to different people.

As in Randolph Hearst (the newspaper magnet),
or fridge (magnet)?🤣

Could a packet of nails ect. be stuck on to an old jumper?

Forget the frock coat, cut and hem/elasticate old joggers below the knee to make breeches, white school shirt, waistcoat on top (from charity shop), piece of lace (charity shop again) to make frilled cuffs and ruffle for front of shirt. Tuck an oldish book under his arm and it's Sir Isaac at work.. you may have to buy the wig but they're not too pricey on Amazon

PS
Stick an apple to a baseball cap and be done with it? The costume could double up if they introduce a William Tell Day.

I agree that it is ridiculous.

Many children are going without breakfast and their parents can't afford heating. Uniforms are expensive too, so asking families to provide a costume for one day is insensitive at best, and insulting at worst. I don't see how dressing up for World Book Day helps with education either. Get them to prepare a talk about their favourite book, or draw their favourite character. Take them to a library. Get an author in to talk to them about how to write a story. All of those things (and there will doubtless be better ideas from teachers ) would do far more to encourage reading than going to ASDA to buy an over-priced off the peg costume instead of food, or expecting mums to add to their workloads and find space in their budgets to produce something home made. Why not let the children make their own costumes as an Art project if it's really supposed to be useful?

I’d just dress him in black joggers and T-shirt, cut out a large magnet shaped piece of cardboard to drape round his neck, and paint that red with black ends.

Our charity shop sells loads of ‘ people costumes’ and all the Disney stuff at £3 each
It’s well worth looking in charity shops for cheap options