In BV - we hardly know about caste of characters except in case of Gopal. I don't think it is about caste. They did show Ganga thinking that this friendship will be a distraction for Mannu as his board exams were coming. The reason seemed right (Pooja-Mannu seems to think themselves as girlfriend- boyfriend and perhaps included I love u also in their conversation) but her handling of the situation was wrong. She should have had a heart to heart talk with Mannu - explaining why she thinks this friendship will distract him etc. instead of warning Pooja and her mother. Similarly - J may have slapped Mannu at the spur of the moment - but this is not right approach to make a teenage boy understand especially when he is already seething with resentment towards his parents.
Mannu - Pooja may seem to be doing what Jagya-Ganga did before - falling in love and marriage against family's wishes, but there is one major difference - both Jagya and Ganga were adult and had the right to at least choose their partner while Mannu and Pooja are too young to even think of marriage now.
J and G trying to come close to Mannu through Pooja will not be correct approach - it will seem to validate the feelings of Mannu towards Pooja or using Pooja for their own benefit (of again coming close to Mannu). J and G have to swallow their ego and try to make Mannu see the truth by their actions. Getting angry or slapping Mannu will only aggravate the problem. When Mannu said - he is Ratan's son and he knows only this way of talking - I wished any body asked him - did he think Rat was this rude uncivilised person? Jagya and Ganga should approach Bimala to make Mannu understand how and why he remained away from his biological father.