IGNOU conducts classes on weekends only and there is no such attendance criteria there but BCA consists of two parts - practical and theory both so theory part can be done without classes but complete guidance should be there for practicals as your presence in all these practical seminars would enable you to clarify your doubt in relation to practicals. Also carrying out practicals independently would boost up your confidence level as well so as per your own interest you must attend the classes.

Also, as per my personal opinion I would suggest you to attend the classes if you are not taking any coaching or even if you are taking  then also at least try to attend 1-2 class in a month.

Good Luck!!!