Ad agencies
Is there any advantage to employing an advertising agency. . .or will that only eat up a budget that would be better spent advertising individually?
It would depend on your goals for an advertising campaign - and your level of marketing knowledge.
If you are looking to do a brief advertising run, you may be able to place yourself with little fuss. However, if you are looking to place a long-term campaign, potentially in several mediums, it could behoove you to retain an agency's expertise. Some schools prefer to use an agency for the sheer convenince factor. Others choose to use an agency, because marketing simply isn't their forte.
An agency should have years of experience negotiating rates, placing effective schedules, and tracking results. A direct-response agency is often the choice of schools, as the trackability and measurability allows the schools to set and follow cost-per-lead or cost-per-start models when developing their budgets.
If you are looking for an agency, approach it as you would any business partnership. Ask for referrals, learn about the history of the agency, and find someone who knows the career college sector - not just advertising unto itself.