Start putting up flyers for open climb days/times. Put happy, fun, cool pictures of people at the wall climbing. Get more people in the doors. Also you need to build up a presence and status among a majority of the student body. They all don't have to climb, just know you're there and support the idea of having a climb gym. People get annoyed easy when authorities close student resources even if they don't use them. Unfortunately a few hundred to a thousand emails from students getting halfheartedly upset about them shutting you down means a lot more than 20-40 people writing passionate emails. The school is in it for the majority of students not a very small portion. You need to show them it matter to a significant portion.
Maybe try to put together a few competitions and advertise for them. Do multiple grades (beginner, intermediate, advanced) so less advanced climbers can still win prizes, have fun, and get involved. Make sure to advertise beginner grades in the competitions if you have them. Go to any outdoor retailers or climbing stores in the area, tell them about the situation, and tell them you are trying to put together a competition to raise awareness and boost the number of climbers. You could probably get a few places to donate gear for prizes. Put their business name on the comp posters and put up a sign at the comp saying they donated. They get cheap advertisement to their target audience and long term they boost sales. It's a win-win. If you can advertise good prizes for comps you'll get more people in the door for the comps which in turn means more people long term. This also boost the reputation and standing with the student body. Next talk to the groups you do parties for. Get them to write up a letter for you saying they enjoyed their times climbing there, how it was a great group event, and how upset they would be if it was closed down. If they go to shut you down you can show them the letters and maybe even get a few groups to back you in keeping the wall open. Again this shows how they are hurting a great student resource that a significant portion of the student body takes advantage of not just a the small core members. I would even start calling up these groups and offer them cheap/free rates for a group night for a while to get more people in.
Another idea is to talk to the school and local papers. Get them to run a story on you. They love this stuff usually. Our school's paper would usually run a story about our organization once a year because for them it was quick easy story and it always front page material. Climbing just plain looks good in pictures and to the general public that doesn't climb it is an easy "Oh that's cool!" kind of topic. Get them to take a cool action'y picture of someone climbing or taking some yourselves and offer them up for the article. If they run a story make sure they mention open climbing hours, group events, competitions, and any previous comps your team won.
Above all keep pumping safe practices. A few people getting hurt when they are on the line about shutting you down is just the little push they need to boot you. That and no one wants other climbers getting hurt. That said if you haven't had anyone get hurt in a long time make sure you bring that up if they try to shut you down. Insurance is a huge issue for gyms and if they know people have never been injured at the wall and you are Nazis about safety it will mean a lot.
Also keep us posted!