Our district has approximately 8,000 students. All can apply for an email account provided for them by the state department of education. The account is theirs as long as they are enrolled in the school system. They have to receive one hour of training and sign an acceptable use policy, which has to be co-signed by their parent(s) or guardian. Students using the state-provided accounts don't necessarily have to use a client-based email program, but can access their email via a web-based interface.
An alternative to consider is one provided by http://www.gaggle.net. I've used it before with VERY good results. Gaggle email can be configured for varied levels of permissions and has been around for at least 8 years. The free version (with ads) is fully functional. The paid version is very affordable.
Thank you, Charlie. That's very cool that the state provides the accounts. Our district uses gaggle for certain business education students, but we (those of us lowly employees in the schools) want all students to have email access. The problem is that we have 11 high schools, and just two of those--the one where I work and another--have 5000 students total.
In the Dallas Independent School District student e-mail use is up to the classroom teacher and the district conducts workshops on the use of ePals for student use. Personally, I prefer and use Gaggle.net. Once your school has an account it is really easy to manage student accounts and it is filtered.