We have participated in the GSoC program for seven years running, in 2007-2013.
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In 2012, 5 team members mentored 4 students. Each mentor was also the co-mentor of another task, so the backup mentoring was still working well. 3 students passed, and one failed at mid-term. One student is still contributing to the project and volunteered to be a mentor for GSoC 2013, which is really awesome. Based on discussion with other projects and on our experience, we are considering merging student code earlier in the GSoC process and have modified our processes accordingly for the next year.