Part of auto insurance is your potential liability cost. I'm personally in the same boat as you, no infractions, same car, and my insurance went up (though less than yours). I live in California, in an area where I'm likely to hit a Mercedes (or some other luxury thing) if I hit anything. Lots of people in my area buy expensive cars and as a result I pay significantly more for the same liability coverage as I would in a lot of other zip codes.
Add to that normal inflationary pressure on auto-body repair shops and you have increases even if you, personally, don't present any new risk.
In response to the discussion below I'm revising this point.
There are loads of factors insurers consider beyond your claims history, driving record and good student status. Maybe a lot of people in your neighborhood are buying Mercedes or some other expensive vehicles. Maybe your newly attained age triggers a different risk evaluation in their models. Any number of criteria related to you could have changed apart from your personal driving record. The carrier has a pool of people and the ability to price certain criteria related to you. The insurer's overall pool needs to be profitable and sometimes that means higher risk people are subsidizing the cost of lower-risk people, and sometimes the opposite can be true. Every part of the risk spectrum will generate claims, claim expenses aren't exclusive to the high risk segment of the pool.
When an insurer increases it's prices that means that relative to the expected claims of the pool, or some piece of your specific underwriting classification, it's not generating enough top-line revenue. Maybe this price increase hits everyone in the pool, maybe it's only the higher-risk or the lower-risk people, maybe it's only the folks with big limit coverage, maybe the student discount gets smaller. No matter the adjustment, the carrier has determined that it either can or should generate more revenue and/or shed some individuals from some facet of its pool.
No matter your personal driving record and perceived risk category, if you get an 8% increase you should go shopping. It's likely that some other carrier wants someone in your underwriting category more than your current carrier does. An 8% increase from your current carrier indicates they expect some number of people like you to leave.