I get extremely irritated when teenagers and adults use incorrect grammar, because they REALLY should know better! This post reminds me of all the teachers I had in school who didn't use correct grammar... When I was in kindergarten, I got sent to the principal's office for correcting the teacher! (I was correct too... lol)
I know nothing about teaching and honestly it wouldn't be the profession for me because I think I would be terrible at it, but I know it must be very frustrating to have to deal with this. I'm sure that if you continue to correct them, and maybe make them write out sentences a few times (maybe 10, not 100

) then perhaps eventually they will come to understand what you're trying to teach them. Good luck!