The exemption for being in college works like this: as long as you're actually enrolled and attending a recognized higher education course, you can request a suspension or postponement of military service. But there's an important detail here - it depends on which year of your degree you're in and your country, because this varies between Portugal and Brazil, for example. In Portugal, you need to present proof of enrollment and attendance at the educational institution, and they usually ask you for documentation from your college showing that you're in good standing with your studies. The best thing is to go talk directly to the recruitment center that sent you the notice, because they'll explain exactly what the procedure is in your specific case and what papers you need to bring.
The process generally involves requesting a temporary suspension of service while you finish your course, rather than a total exemption. There's also the question of timing - some courses have periods where the suspension is automatic if you prove you're enrolled, but in other cases you have to request it explicitly. One thing a lot of people don't know is that after you finish college, the service can show up in your life again, so it's not like you get exempted forever.
What's your exact situation - what year are you in? And what's the country where you got the notice? Because depending on that, what you need to do changes quite a bit.