If you can, talk to the athletic director at the school and ask if you can have a field pass. Shooting in the crowd is going to bring you, like a bunch of other people have also stated, unwanted hassles.
I'm a senior at my high school and just received a field/media pass to the sporting events, including away games. It may be a lot harder for you to get one than someone like me who's a student with no intentions of allowing the pictures taken to leave my hands, but it's worth a shot. Maybe it's because I'm young, but it's a great feeling to be able to walk onto the field with your photography gear and not get stopped, at least for more than it takes you to show the security your pass.
About the thin skinned professionals on the field; I wouldn't worry about them. For me, if they confront me I will simply tell them I am taking pictures for my own experience and practice. The pictures will are not going to be given to the school, any parent, or any student playing (even though most of them are good buddies of mine). You, planning to give away your pictures, might get a harder time from the guys who are trying to hold a job, whether it be a second job or not.
I say don't bother anyone and there should be no reason for them to bother you. If they confront you, be kind and to the point and go back to doing your own thing. Have fun, good luck.