Steamy games at school produce inappropriate characters (7)
I believe the answer is:
mistype
'produce inappropriate characters' is the definition.
(typing the wrong characters)
'steamy games at school' is the wordplay.
'steamy' becomes 'misty' (synonyms).
'games at school' becomes 'PE' (Physical Education class).
'misty'+'pe'='MISTYPE'
(Other definitions for mistype that I've seen before include "Write incorrectly (on a keyboard)" , "Hit a wrong letter on keyboard" , "What poor secretary may do" , "Use the wrong keyboard keys" , "Select wrong character" .)