Overtaxing school lessons (9)
I believe the answer is:
straining
'overtaxing school lessons' is the definition.
'straining' can be an answer for 'overtaxing' (thesaurus). I'm unsure of the 'school lessons' bit.
'overtaxing school' is the wordplay.
'overtaxing' becomes 's' (I can't explain this - if you can you should believe this answer much more).
'school' becomes 'training school' (training school is a kind of school).
's'+'training'='STRAINING'
Can you help me to learn more?
(Other definitions for straining that I've seen before include "Making a great effort" , "Exerting much effort or energy" , "demanding" .)