I've used coworkers and friends as references. I have friends who still continue to use me as a reference over past managers. These are friends whom I've worked with in a professional or classroom setting. In both situations, I'm able to speak to their ability to act professionally and/or work in a team.
Honestly, none of my references have ever been contacted and I've never been contacted so I can't say if this has had a positive outcome in a job search or not.
As for asking a teacher, this will depend on high school vs college.
Many college professors/teachers work summer hours (teaching is not the sole responsibility of professors). You can definitely try calling and leaving a voicemail or email them! (if you don't have their emails, your college website might have it available)
For high school, this is a bit trickier. You could always drop by the school and see if anyone is around, honestly, even in HS, teachers are there sometimes during the summer. Or you could call the school, either a receptionist will answer and you can ask if so-and-so will be in any time soon and if not how to contact them if it's okay or a prerecorded message should play telling you the summer hours of the school.