The MAA server has been using 2.8 for some time now and 2.8 should be solid.
Many others have been using 2.9 this past fall term, and there have not been major concerns that have not been addressed (as far as I know).
I've been using 2.9 with additional bits and pieces of the develop branch pulled in. I don't recommend you do anything like that if you have to go through your IT people to work on the server, but I haven't had trouble and it's made me feel good about the next option below.
Version 2.10 is going to be released sometime soon. Geoff Goehle is preparing to create a 2.10 candidate branch from the develop branch, and it sounds to me like it may be only a matter of days before this happens. At that point, several of us will test it out as much as we can. And then when there are enough green lights, version 2.10 will become the official master version.
These are all options for you, but if you can stand the uncertainty as to when 2.10 will officially become master, then I'd recommend waiting for it. Among many nice added features, a lot of web accessibility enhancements went into it from one of the code camps this past summer, which should please your disabilities services staff.
Whoever does your IT stuff though, upgrading versions is pretty painless using git, updating the OPL, and updating database tables in the admin course. So even if you chose 2.8 or 2.9 right now, at the end of the next term/semester you could easily upgrade then assuming 2.10 passes all vetting up to that point.