As public health advice has been updated, the starting position now is that pregnant employees need not be excluded from returning to working at school.
However, each pregnant person is clearly in a unique situation (in relation to where they work, their health, their pregnancy etc.).
As such, if your employer has not already contacted you to discuss undertaking an individual risk assessment and you are concerned about returning to school, you should make contact with your employer straight away. (You could contact your head teacher directly, or may want to contact HR.)