Tips for Doing Remote User Research Interviews
1. Make it as close to face-to-face as possible
A face to face interview is ideal, but in your case it is a restriction. Its best to interview over video conferencing tools. The benefits are being able to connect emotionally with your interviewee easier (which puts them at ease), being able to read facial cues, hearing inflection in their voice, and being able to follow up on specific responses they provide.
If video conferencing is not an option, then use phone so that voice inflection can still be heard and you can follow up on responses. A write in response would be a last resort considering what you would be giving up.
2. Write Out Your Questions, But be Flexible
Having specific questions to ask is great and gives structure to the interview. That said, a key advantage of interviewing is that you will get responses you hadn't planned for. Roll with it. Ask people to expand on what they said and explore info that you hadn't considered but would be valuable to designing your app. The term for this is semi-structured.
3. Make the Interviewee Comfortable
You are asking questions of people you don't know. They are sharing with you. The whole experience is rather personal. Make sure you make the person feel comfortable so they can respond honestly and openly. Ask small, easy questions like how's their day going, who do you teach, and what they enjoy about teaching. By setting them at ease, they will open up more and provide you with a more informative response to your questions. Seeing faces, even if its video, helps put people at ease.
4. Start Recruiting From Your Network
Since this is a class assignment, the target group is teachers, and you only need 5-10, I would recommend recruiting from your personal network. Ask friends and family if they know former teachers they keep in touch with or are friends with. Look to social networks on Facebook or Linkedin for teachers you may know or your connections may know. The connection makes recruiting easier and gives you a head-start on putting the interviewee at ease.