Mailing lists
You can buy mailing lists to undertake your own mailing yourself - which is often an ideal way of undertaking small test mailings.    However do remember that when you mail through a specialist mailing house you will save considerable sums on postage.   

At the simplest level, postage stamps cost more than the postage charged through any form of Royal Mail account.  Additionally bulk mailings attract substantial discounts from Royal Mail and its increasingly active rivals.

Solo mailing

Sending your material out on its own to the addresses you select is known as "solo mailing".  

Brochure Mailing and Shared Mailing

As an alternative, where you are mailing schools you can opt for "brochure mail" and "shared mail" - approaches in which packages of promotional material from various companies are sent together to the selected schools.   These approaches allow you to mail for far less than the cost of postage.

Finally - testing. 

Many firms by-pass this stage in the process, and the result is often a disaster.   You really do need to test a promotion before mailing en masse