It depends what you are struggling with at school because of your eczema. Eg, a friend was allowed to have his top shirt buttons undone so they weren't rubbing on his neck. In other cases you might be able to request that certain cleaning products are not used (it might not always be followed) or that you can have rest breaks in an exam to take/apply medication.
Those are random examples, but it really depends what you are struggling with. You need to identify what may help and talk to the school about it (or ask your parent to), and get medical evidence from your GP.