So, if you think that ads can make a good earning then you are wrong. You don’t earn a lot of money from the ads, but only if you have many users, say about 10k to 100k daily active users, then you can consider it as a real earning.
I understand the problem of the teachers of your country but I don’t think that what they will earn from those ads will be enough. If the app that you are developing is really very much beneficial for the students, then you can ask the teachers to charge a one time (or a monthly/annual) fee for the app instead of depending on those ads. 
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