"Stan The Man" wrote ..
My son's school is looking for vegetable seeds which can be sown after the
Xmas hols and produce something harvestable, or nearly edible, by the end
of April. Seeds could be started off on classroom windowsill (no
greenhouse or cloche to hand) and then planted outdoors; or any seeds
which can be put straight into their outdoor veg plot... Any advice
appreciated.

Lettuce (especially the red varieties) and Mustard & Cress spring to mind.
You do plant onion seeds early but they won't be cropping before next
summer.

--
Regards
Bob Hobden
just W. of London