A lot of schools don’t want students (or teachers for that matter) to use external email system (such as hotmail, gmail, yahoo, etc). There are several reasons for this. The main one is that students and teachers opening up attachments can bring virus, worm or other nasty stuff into the schools network, who don’t all have the best in anti-virus protections (a lot of schools go the cheap route and just use stuff like AVG’s free-ware version, or malwarebytes/super anti-spyware, etc). Another reason is that administrators think that students will send email messages back and forth (while not realizing that this small boxes in their hands can send text messages back and forth), and teachers might be reading email when they are supposed to be teaching class.
It’s variable though. Some schools I know allow external email, some have their own internal email system (some have admin and teachers only, some include students as well), and some don’t allow any email…or allow it for admin only, or admin and some teachers. It’s a mixed bag.