Flies in the Classroom

I'm having an issue with flies in my classroom. I have searched and searched for a way to get rid of them. I have a fly trap, essential oils, sometimes a fly swatter, but it's getting ridiculous. Has anyone had this issue before? How did you get rid of them??<br><br><br>Edit: Thank you all so much! I have made a trip to the dollar store and bought some vinegar and wine. I'll post and update and let you all know how it goes.