Tough to answer in full without knowing what your service is, but there are plenty of ways to get in front of college students.

A few ideas:
- Build a blog with useful/interesting content, share on social, build an audience
- Incentivize current users to invite their friends through a referral program or a campus rep program
- Find sites where college kids spend a lot of time (think: Reddit), or posts relevant to your service and get involved in the conversation
- Are there on-campus clubs or groups that would find your service particularly useful? Try reaching out to see if they'd be willing to give it a try

Let me know if you want to hop on a call & discuss more ideas.