"New" law (i.e. Lithuania related lessons in Lithuanian, equal Lithuanian exam) is not aimed to hurt minorities or to assimilate them, instead it is aimed to help with integration, and solve some other issues. It is not fun, when you go to hospital and you are asked to speak Polish or at least Russian at reception desk, or when state employee admits that she can't read in state language. It is not fun when person, who immigrated from South America, manages to learn state language in 3 years way better, than person who lives here for 40-50 years. Sounds like twilight zone, but these are examples from my personal experience.
It is obvious that lacking skills in state language can be a huge problem for gaining higher education and career. Problems starts outside of area near Vilnius, because most studies are in state language at universities, and huge amount of already complex information received there are made even more complex due to language barrier. Same with career: it is hard to get a good job outside of Vilnius district and Vilnius itself (to some extent), if you have problems with state language. Mind you, that same applies to Lithuanians. Minorities not wanting to learn state language are simply isolating themselves and lowering their opportunities in whole state. Brighter students understand that easily and don 't oppose the idea, because they think they will need state language anyway.
That law is aimed for solving such things. The only problem is terms, whose are nonsense. I think they should introduce changes only for new generations (i.e. new education program for lower grades) and not for current students, because it is impossible to learn so fast, and they need to learn a lot. I suppose there will be changes made to law for giving more time for adaptation as I have already said: terms are nonsense. Current government is notorious for making quick overnight decisions without evaluating them fully. :)
If that law has good intentions, why are people protesting? Well, that thing is greatly escalated by some politicians, whose simply manipulate other people. They have power in Vilnius district and some other districts near it. High corruption, high unemployment and not much perspectives there. Media isn't independent there too (i.e. politicians assigning money for local newspapers and so on). In other words, politicians can influence local people a lot and in a way they need. That is very bad, because it makes tense relations not only between Lithuanians and Polish minority, but between Lithuania and Poland too.
Something new :)