I brood different aged duckings together all the time to safe space ( and time cleaning ) At the most its usually ten days difference for young ones including those just hatched- but i will at times put ducklings of a greater age difference together without problems. You might just need to keep the second lot separate for 2 days until they are up and about and eating well so they are well able to get around the brooder with the bigger ones. Good luck with the eggs - Im sure everyone at the school is very excited about them.