All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
There is nothing wrong Islamically about the mere debate of these matters in the classroom; let alone it leading to Kufr.
But it is not permissible for you to remain silent about what you hear of evil. It is not necessary in a debate to state a verse or a Hadeeth. You can use the method that can be acceptable in the debate, such as clarifying the harm caused by such acts, and that harm is forbidden in all heavenly laws, and by the wise people.
You can provide evidence of some examples in the reality of many countries that criminalize those who do so, and some other good methods. But you should not remain silent.
It is obligatory to abandon such meetings.
Allah Says (what means): {And when you see those who engage in [offensive] discourse concerning Our verses, then turn away from them until they enter into another conversation. And if Satan should cause you to forget, then do not remain after the reminder with the wrongdoing people.} [Quran 6:68]
This is as long as one is not excused for a necessity, as there are some rulings that apply to necessity.
For more benefit on the steps for denying evil, please refer to Fataawa 92398 and 1048.
You must beware of whispers on Kufr and apostasy. In principle, a Muslim remains a Muslim and he is not judged to be a non-Muslim (Kaafir) except with an Islamic evidence. The scholars wrote in their books about the matters that lead to apostasy.
It should also be noted that there are some conditions that must be met in declaring someone as a Kaafir,
For more benefit in this regard, please refer to Fatawa 8106 and 87963.
If we presume that someone had apostated from Islam, then if he repents and re-embraces Islam, then this will be accepted of him.
Also, if a Kaafir –whether he is originally a Kaafir or an apostate –performs the prayer, he is ruled to be a Muslim, as stated by the scholars of the Hanbali School. They also stated that apostasy invalidates ablution; this is as well the view held by the scholars of the Maaliki School.
With regard to the repetition of a verse or a word in the recitation of al-Faatihah, if it is for an excuse, such as doubt, for example, then there is nothing wrong with that. However, if it is without excuse, then it should be avoided because some the scholars are of the view that the obligatory continuity in the recitation of al-Faatihah is interrupted (in this case).
The book Tuhfatul-Habeeb in the Shaafi'i School of jurisprudence, reads:
“If he repeats a verse from the very al-Faatihah, then al-Qaadhi Husayn said in the Fataawa: “If it is repeated very frequently, in a way that there was a long pause, then he should start anew.”
Al-Bayaan reads: “If it (what is repeated is) is the first verse or the last verse of the Faatihah, then this this does not affect. However, if it is in the middle of al-Faatihah, then the analogy requires that it is as if he recited another verse during al-Faatihah. If he did so deliberately, then his recitation is void, but if it was out of forgetfulness, then he should continue.” [End of quote]
The same thing applies to the words of the Tashahhud. Some scholars are of the view that continuity in Tashahhud is obligatory. Az-Zarkashi said in al-Mafthoor: “The continuity is obligatory between the words of al-Faatihah as well as the words of the Tashahhud, as stated by al-Mutawala. Ibn ar-Rif’ah said: “It is by analogy to al-Faatihah.” [End of quote]
Allah knows best.