Firebending abilities aren't mana abilities. They use the stack and can be responded to.

Multiple instances of firebending on the same creature trigger separately, each granting you the appropriate amount of mana.

"Firebending N" is a keyword that represents the ability "Whenever this creature attacks, add N {R}. Until end of combat, you don't lose this mana as steps end."

Mana from firebending abilities isn't lost until you leave combat and go to your second main phase. It can be used at any time during combat, even after combat damage has been dealt.

Any other mana you add during combat will still be lost as normal when moving between steps of combat.

You must have a Lesson card in your graveyard while you attack with Fire Nation Cadets in order for the firebending 2 ability to trigger. If you put a Lesson card into your graveyard after Fire Nation Cadets has attacked, Fire Nation Cadets will still gain firebending 2, but the ability will not trigger.