You can do the following
1. Where college students most hangout/eat at peak hours not just within college but outside the college (that favorite place of students near the college) then create small sessions with permission of the place owner and give a presentation in a cool fun way about your app, food place is a great way to promote
2. Find out the largest group chat students are part of it and then make a video and share in the group it can be multiple group chats.
3. Ask teachers/lecturers to talk about your app in their classroom for 2-5 minutes, if you are comfortable and they are ready to support you.
4. 4. Go to student hostels if the college has one and with permission of authority promote your app thereby just gathering students for 5 minutes in the evening time.
All these really require effort communication and patience.