Christians are no longer held to the strict laws of Sabbath (an argument can actually be made that Jews were never held to this strict law, either, but that they made it stricter than necessary--but that's beside the point).
I have had jobs in the past where I was required to work on Sunday. To ensure that I still had a "Sabbath day" (day of rest), I chose some other day of the week as my rest day. I think this practice is well within the intention of Sabbath rest, as outlined in the Bible.
But you're asking specifically about a certain task, whether it is work or not. I think Jesus demonstrated that the standard for Sabbath day activity should not be "work,"--as he did several acts of "work" (including harvesting grain, and healing) on the Sabbath. Rather, I think the more important principle is "Remember the Sabbath day and keep it Holy"--that is, use the day for God's glory.
If, for you, that means no studying, then don't study. If for you, studying includes attending a lecture, then don't attend that lecture.
For many people, studying is very stressful, and distracting... for those people, I would think choosing to avoid study as part of their Sabbath ritual is a good idea. For others, studying may be a relaxing thing. I'm not in school, but I find it relaxing to read--and I rarely ever read fiction. That means that almost all of my reading is "study" of some form. If I therefore never "studied" on Sabbath, I would be avoiding one of the most relaxing activities I know.
What it boils down to, I think, is your own personal conviction.
You did the right thing to refuse the lecture, which would have gone against your convictions. If you decide that your convictions should change, I'd say you're also doing the right thing, as there is no "right" answer in this case.