Number at school help to turn over slightly repellent material (9)

'slightly repellent material' is the definition.
The definition suggests a singular noun which matches the answer.

'number at school help to turn' is the wordplay.
'number' becomes 'ten' (ten is a number).
'at school' becomes 'ga' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'help' becomes 'maid' (help can mean servants).
'to turn' says the letters should be written backwards.
'ten'+'ga'+'maid'='tengamaid'
'tengamaid' back-to-front is 'DIAMAGNET'.

'over' is the link.

Can you help me to learn more?