Answer: The idea of saying specific du'as or specific surahs at specific times, or the idea of saying things in congregation, requires evidence from the actions of the Prophet ﷺ and that which was transmitted by his companions.
Yes, it is possible to teach people by saying "repeat after me" for the purpose of learning. However, the idea that there is a kind of collective act of worship where everybody gathers to read a du'a in congregation to begin or end a gathering is not something that we know the Prophet ﷺ did. This practice requires evidence for it.
The second part of the question relates to commanding good and forbidding evil. The Prophet ﷺ said:
مَنْ رَأَى مِنْكُمْ مُنْكَرًا فَلْيُغَيِّرْهُ بِيَدِهِ فَإِنْ لَمْ يَسْتَطِعْ فَبِلِسَانِهِ فَإِنْ لَمْ يَسْتَطِعْ فَبِقَلْبِهِ وَذَلِكَ أَضْعَفُ الْإِيمَانِ
"Whoever among you sees evil, let him change it with his hand. If he cannot do so, then with his tongue. If he cannot do so, then with his heart, which is the weakest level of faith." (Sahih Muslim 49)
Changing with the hand is for people who have authority. For example, if you're the principal of a school, you have the authority to stop something directly. So, you stop it.
فَإِنْ لَمْ يَسْتَطِعْ فَبِلِسَانِهِ
"If he cannot, then with his tongue."
This applies to people who have the requisite knowledge to challenge it verbally. For instance, they may go to the principal, who has the authority to change the situation, and say with respect and kindness: "It seems to me that this action might not be correct. I would appreciate it if you could look into it and perhaps put a stop to it."
However, this requires knowledge. If a person does not have the knowledge to challenge it, nor the authority to change it, then that person only has to hate it in their heart—and that is the minimum standard of faith.
The next point to understand about commanding good and forbidding evil is that you can only do so if it will bring about greater good or stop greater evil.
It is not permissible to verbally challenge something if doing so makes the situation worse. For example, if you go to the principal and say, "I believe this action is not correct and I think it might not be what the Prophet ﷺ did," and the principal becomes angry, suspends you from school, or gives you detention while continuing the same practice, then it is not permissible because you are not improving the situation.
Instead, you may be able to raise the issue with the school in a gentle and wise way. For instance, you could say: "I was wondering if I could ask about this collective du'a that we make. I couldn't find an example of the Prophet ﷺ doing it and wondered if you had an example you could share with us." If they reply that there is no example, you could then say: "I wonder if it's a good idea to continue if there is no evidence from the Prophet ﷺ."
However, I strongly advise that you only challenge something with appropriate knowledge. This includes researching the issue to confirm that the Prophet ﷺ did not do it. Additionally, when you challenge something, ensure that you do not make the situation worse. This might involve asking an adult or parent to speak on your behalf.
And Allah ﷻ knows best.
— Answered by Ustadh Muhammad Tim Humble
