no there is no church law that says a parish cannot choose to offer catechism classes, bible studies or any other program of interest to adults in the parish on Sunday before or after Mass.
That being said, depending on your area, attendance might not be very good. Here for instance the tradition of Sunday as family day is quite strong (a good thing) and it is very hard to get people here even for something mandatory like a Confirmation parent meeting, let alone a voluntary class.
Many parishes, including ours do have Sunday classes. We have a Confirmation class for adults, and since it is a systematic exploration of the CCC it is open to all adults of the parish, and each year we do have a few participants who have already been confirmed but are interested in a formal CCC class. We also have RCIA on Sunday morning for adult, teens and children, (all these are in both English and Spanish) and a lectionary-based CCD class for children who cannot come on Saturday to regular CCD. There are also a couple of other offerings Sunday morning, including a men’s group, and some of the bible groups meet on Sunday at various times.
But in total, participation out of a parish of 3000 is only about 50 taking all of these together. If this parish wanted something on Sunday, people would come.
We have advertised and invited many, many times any program you can think of, on Sunday or any other day of the week, and response is nil. This parish can have whatever it wants, but they have to want it.
I will also point out that resources in many parishes, in terms of room, resources and people are stretched to the limit on Sunday and it is also the hardest time to find volunteers to staff these programs. If you feel it is the way to go in your parish, be among the first to volunteer.