If possible, I would try to ensure that all the recorders were of the same make. If you bulk order from a supplier, this wouldn't be problem. Even two good recorders of good quality may well not be in tune with each other. I believe that it is worth taking the time to tune instruments properly, but this may not be practicable under school conditions.

An electronic tuner will tell you if a recorder plays in tune. I know some people don't believe in them, but I know that they really do exist, because I own one and find it very useful.

It occurs to me that a penny whistle might be an alternative. I just bought one in Bb today (I've got one in C and one in D). It only cost 8.90 Euros and the smaller ones (in F and G) cost less than 8 Euros. One could probably get a discount if one bought them in bulk. It's harder to play accidentals on a penny-whistle than on a recorder, but it's not really that hard.

After shopping around, I recommended that the parents of my recorder students buy a two-piece plastic recorder made by Mollenhauer. It cost about 20 Euros. It can be washed in the dishwasher, by the way, but I hope this won't be necessary. This is probably too much some parents.