Hi Sebastian,
At the moment, we don't have a way to reset/release an account. If we did, it would obviously need to be done by the owner, as only they could say they are happy to do it! But it is not on the plan for now, and here is a reason why for the student case...
One thing we are hoping mbed enables is that the student can own the hardware, rather than "the lab" owning it - this way, they are free to use it in any projects, coursework, hobbies etc. What we've seen a lot over the years is development boards that are restricted to use in the lab, and "student" versions of tools that are only available with the lab computers. And this really gets in the way of use outside a scheduled lab (which is where you *really* learn to use something!).
The mbed tools mean the software is usable from anywhere, not just the lab, and hopefully the same is true with the hardware - it is not a $1k development board so can hopefully be owned and taken home/reused by the students. That means it can all be applied outside a lab too.
It would be great to get feedback on the realities here. In summary, our experiment in this area is to see how things change if you encourage kitting out the students with hardware/tools, rather than "the lab" (so far, the results look very promising!)
Simon