How do I get a coordinated frequency pair assigned for my repeater

How do I get a coordinated frequency pair assigned for my repeater.

I am thinking about putting up a 70cm repeater up on my High School, where they have a very tall roof.

By the time It is ready to put it up, I want to make sure a frequency pair is reserved for the repeater.

Who do I contact / where can I get a frequency coordinated for the repeater?

That may require some effort by your club to check all the coordinated repeaters to see which are still up and running. It is not unusual to see repeaters still registered long after they have gone of the air. If that is the case you should be able to register that previously used frequency pair for your club repeater (attempt to contact the registered owner of the former repeater to see whether it is permanently gone or just off the air for maintenance/repairs).

Even if all the repeaters in the region are still operational (just not used very much), you may still be able to get coordination for your club repeater but it will take more effort. You need to plot the reach of your planned repeater (coverage maps) and then show that it doesn't overlap with any other repeaters sharing the same frequency pair you are applying for.
Your regional repeater coordination group should have guidelines for that process (which may involve running your repeater uncoordinated for some period and checking for interference reports).