Ok, that makes it easier and harder.
Start here: "We've calculated revenue per campus as well as cost per school/cost per student/revenue per student and all that..."
There are some businesses where the economics get better with scale (search advertising for example) and some where scale doesn't have nearly the same level of impact (a lot of service businesses, grocery stores, etc) Yours would seem to be more of the latter.
As much as they dislike the characterization, many sales people are coin operated, specifically the profession attracts people who like an easy to use 'scoring' system for how good they are, and sales is great for that since the score is built into the job. Since you feel like a listings manager can handle 35 schools, and there is three of you, why don't two of you start in the role of 'listings manager' and the third hire a sales person and focus on getting student "reps" signed up. Plug into the student financial aid department, they love having part time work opportunities with flexible hours for students. The sales person is going to want to be "hands on" and talk to people directly. Ideally the person you hire here is going to eventually be your VP of sales and will train other sales people, so consider making a deal of equity for work early on.
Challenge yourself to take the people you have, add exactly one sales person, and make enough money to pay everyone salaries. If you can't do that, with 35 schools then you don't have a business yet. Don't be afraid to fire your sales person if they don't deliver. I isn't anything personal it's just a fit thing, either they can sell your product or they can't, either the product can be sold or it can't. If getting advertisers is "easy" then your sales guy will be very productive, make themselves and you a lot of money. Also don't be too surprised if you can't work a very large territory with a single sales person. Getting people to part with their money is an art and it takes skill just like writing code does, respect that.
Always keep in mind the 'big plans' and so when you make decisions make sure they don't cut off future plans (for example don't sign off advertising rights to a third party for the rest of the world or something) but get the core of the business running and tuned, then take it to the next level.