Are there requirements for an assessment board´s composition?

No. There are no provisions in the PSEA that require a deputy head to establish an assessment board, or that a board have a certain composition. However, if an assessment board is not properly constituted, and is unable to make a fair decision as a result, then you may be able to show that there has been an abuse of authority in the selection process. This a question of fact which depends on the specific complaint. You would need to provide evidence to show that the selection process was unfair because of the composition of the assessment board.