Here are some idioms to help you express yourself (related to school and education) :

– You shouldn’t be too / so hard on yourself. (To be too self-critical)
– I went all red in the face. (To become embarrassed / To blush )
– It can’t be that bad. (I don’t believe that it is as bad as you say it is)
– You passed your degree with flying colours. (You easily obtained your degree)
– Put your thinking cap on ! (Start thinking)
– When I have to speak in front of a lot of people, my mind goes blank. (I forget everything.)