This is a process that has to be ongoing. If you're only doing it by ear, one thing I've always said is if it feels like you're louder/sticking out, you're probably sharp. If you hear a lot of the wah-wah waves, you're probably flat.

I truly think the absolute best tool though is to have personal tuners for every student and clip them to the stand. Train them to look up especially on longer notes. Use Tonal Energy/Harmony Director to do drones, practice adjusted intonation as your kids get more solid with tuning understandings.

A lot of kids can be told 'you're out of tune' and not understand at all. Having a tool in front of them that they can see/be trained to understand will then help them develop the ear to adjust without it later on.