Is mocking teachers who offered assistance considered an additional sin, beyond the sin of ridicule and imitation? And is it obligatory to advise friends that this act is forbidden?

Praise be to Allah, who guided you to regret the act of backbiting and mocking the teacher. This act is more severe and greater when directed at someone who has been benevolent towards you. Repentance from backbiting requires desisting from the act, regretting it, and resolving never to return to it. Additionally, it requires seeking forgiveness from the person you backbit, unless you fear causing mischief, in which case you should supplicate for them and seek Allah's forgiveness on their behalf. You must also advise your friends to abandon this sin. If you repent and reform, you bear no sin for what they do after your repentance, for the mercy of Allah is vast.