Even if you haven't graduated, it's okay to call a teacher, and it's okay to text them if you get their permission first. If you ever wanted to be their friend on Facebook, however, you DO have to be graduated. I have my band director's cell number for when I need directions to a band function or whatever else I may need. It just depends on the teacher.
Of course. I graduated high school and keep in touch with some of my favorite teachers through facebook. Or I'll go back to high school and visit them. I have their email we were even planning to hang out (girl teacher for this one)
I would think it would be fine. It's all up to you though and make sure your teacher is okay or comfortable with it. If you and your teacher are okay with it, then that's okay.