It's not clear to me if you mean "This summer school could be a crucial step in my medical education." or "...could be a crucial step towards my ..." The meanings are diffferent; the first, that it could be a crucial part of your curriculum during your medical education, the second, that it is something crucial that you could do prior to beginning your medical education.
ain'tt...fun's point is that:
If the summer school is something you do during the medical course you say "in".
If the summer school is something you do before you start the medical course you say "towards".