The first one could mean that you can study a lesson a week, also it's like a generalization, I mean, anyone can study a lesson a week, it can be done The second one means that HE can study a lesson a week

-cuantas lecciones se puede estudiar a la semana? -se puede estudiar una leccion a la semana -puedo estudiar una leccion a la semana? -puede estudiar una leccion a la semana

Both mean that someone tells that "something" could be studied 1 time by week. But the first one is like an opinion, because someone could have had an experience about trying studying something more than 1 time by week and it didn't work, so now that person is saying that something could be studied 1 time by week (based on her/his experience or thinking). And the another one is like an order, but it's being said formally, and not lcompletelly like an order, it's said with respect but someone is saying an order in one or another way. Equally, both sentences generally have the same meaning, because the difference between them is just an ambiguity, but into a daily conversation, both could be interpreted like the same thing, with the same meaning. But If you someday want to say that friendly, or like an opinion/advice, I think you should use the first one :)