I think this is a wonderful proposal. By joining forces and avoiding duplicate efforts, a completely free IT solution tailored to schools could only gain in quality and attractiveness.
However, I won't be available this summer and I think I don't have enough technical skills to act as a mentor. Perhaps some other Debian Edu developer is willing to take on this role.
Dear Frans, thank you for your support.
Because it is already late, I have just added a template project on the GSoC list, as a placeholder:
Now it is up to the people from FUSS and DebianEdu to meet and discuss with each-other about the details of the project and how they would like to manage it.
I could also help as a secondary mentor, but I will also not be available during this summer.
I hope that this project idea will move forward and will be successful.