Hey! I've lived in Mission and loved it, but Kensington is definitely the most convenient for getting to school. It is also really vibrant with lots of other students living there, shops, restaurants, etc. Mission is great, but not being by the train makes it more difficult to get to school. I live in Marda Loop and love it -- given that the 20 goes directly to the school I would say it's even more convenient than Mission. But you can't beat Kensington for convenience.
Mission is closer to 17th ave which is where there's a lot of bar patios and nightlife, so that is a benefit. I found living there for a few years, I got to know a lot of the regulars that were also in their 30s and it had a real community feel. That said, I am sure Kensington has a similar community spirit.
I'll also add that both have really lovely parks and access to the river so are great for long walks when you need study breaks -- it's not all about the nightlife! Quality of life is so affected by the area you live in, so you're right to try to pick out the right area for you Also the Safeway is better in Kensington if that matters to you