Can you help me come up with guerilla marketing methods targeting college freshman

Without more information we can deduct some basics from this:

Who is your target audience: College students. What do we know about them?
- They are busy
- They are willing to invest in themselves
- Most like to party
- .... This list goes on for a while ...

What do you know about their lives as College Students?
- How much does the average grad make? Start campaign ideas based on that.
- How much is the average student loan after interest? Start a campaign based on that.
- How often to college students flunk out? Start a campaign based on that.
- ... This list also goes on for a while ...

So how do you reach busy, invested, driven and (mostly) young people?
I would work from the inside, or their private online networks. Does a college still use their own version of Facebook (before Facebook was around)? If so, find a way on there. Otherwise why not find a student in a particular college to help you market? I'm sure you can't just walk into a college and start signing up people. But another current student could. (could be a good addition to your team, too).

Look for Google+ groups, Facebook groups, student forums, Twitter hashtags, and other social sites where young punks who cut across our lawns hang out (LOL. Joking about the young punk thing.)

If you're ok with non-students, you have LOTS of resources. Here are two from just one website I can think of off the top of my head.
- The place to get Alpha and Beta users to test your app/website
- r/SideProject - A community for sharing side projects and startups

Post flyers at the local colleges. They have announcement boards.

Also, perhaps try to work out a deal with the local businesses around college campuses where you give them flyers to leave at their register and in turn offer to have their ad on your flyer as well

I read a post the other day that fits this to a T. The suggested having desperate students actually go into related classrooms before class starts and put the fliers right on the desks. By the time anyone complains several of them are already stuffed into people backpacks.

Are your plans to make money with your app? If so, you might want to rethink the targeting of HS seniors and college freshmen unless this is strictly monetized through advertising.

Steve