I have done it several different ways depending on the size of school and number of alumni playing....
1. Have played alumni against current team.
2. Had current players and alumni on mixed teams with both alum and current players on same team
3. Had two games. Recent grads played current team and then played an oldtimers game between two alumni teams.
I never did it as a fund raiser. I was meant to build community support and have a little fun. You could easily do a hit-a-thon, HR derby, etc. before getting started or between games. Selling food is also a good money maker.