Have you tried talking to your teachers about it? I know it can be awkward/embarrassing, but they need to know what's going on. They might not fully understand what stuttering is. Once they're aware that you know the answers and just need more time to articulate them, they should become a bit more patient and supportive. Also, I know it's easier said than done, but remember you can always use synonyms to help you get past blocks. For example, if you can't say "pretty" and you feel like you might be interrupted, it can be worth switching to another similar word ("somewhat", "kind of", or just "bad"). That helps me sometimes.