Hi there, based on you situation and my experience, one of the best marketing methods is offline promotion, around or in each schools. Run ads in/around the schools is a great choice.
Before starting to promote your app, you can buy positive reviews to increase the download rates. Because 88% users trust reviews as much as personal recommendations.
Besides offline promotion, online promotion is also necessary, especially in the app stores, because over 65% users discover new app by browsing or searching keyword in the app stores, and most of them just browsing the top 10 apps, then the best way to promote your app online is boosting its ranking in the app store. To boost app ranking in the app store, the most important part is choosing keywords, because just like seo, the keywords also play an important role in app store ranking algorithm. Besides the competition and traffic of the keywords, the relevance is the precondition.Follow the following methods to boost your app ranking in the app stores with specific keywords:
Hope those methods are useful to you.