May I ask that you consider using a sso instead of a captcha? G+ would be a good alternative as quite a few higher education units have it implemented for students to use.
Captcha’s are full of inaccessible interactions, and when they’re made accessible, bots can then interact with them. A relevant xkvd comic on the issue.
The problem right now is that Dragon, and other voice recognition software, users are seen as “bots” in some of the new captchas while the audio captchas for the visually disabled are not great alternatives.
