Probably one of the most easy

And most difficult lessons for anyone to learn when learning a new language is to learn to speak up in class.  I know for a fact that every language teacher in the world wants their students to participate, and how else are you going to get practice actually speaking the language when you aren't in the country? You might be thinking "oh i'll do it with someone else later" but here's the thing.  The teacher is (one would hope) a master of the language and its grammar.  If you say something wrong or silly they'll correct you and you might be embarrassed, but think about how much worse it would be if you're talking to a native and made that same mistake when you were speaking very fluently and culturally correct. A whole lot more embarrassing right?  A great example of this happened to me the first time I went to Germany.  I had developed a habit of answering questions (usually about whether or not I wanted to eat something) with the statement "I'm good."  It's a relatively acceptable thing to say in English to someone asking you if you want more food right? I think so, and my host mother asked me that very question in German.  So I translated it directly into German. (translates to "ich bin gut") The next day I overheard some of the other German students who had gone on said trip with me talking about how they had learned just that last year in class (they were one year ahead of me) that the term "ich bin gut" means to Germans: I am good in bed.  In fact the proper way of informing someone that you would not like any more food in German is "ich bin satt" which translates roughly to i'm satisfied (and a danke on the end wouldn't go amiss either).

Now I realize that there are a lot of things (like that one) where you probably won't learn it in class unless the situation is just right, but actually speaking up in class does a lot of other things too.  For instance it might make you more comfortable about speaking the language in general (and more confident too), it'll help prevent you getting into bad habits with grammar or pronunciation, and your teacher will love having you in class which could possibly get you a better grade (*wink wink*).  And one of the biggest things, (in my opinion at least) the all important fact that you might voice the question everyone else is thinking but too afraid to say.  That particular fact is true of any and every class you're in honestly, but it shouldn't be something that should ever happen in a language class since you're taking it to be able to speak the language.  Why the hell would you not want to speak it when you expressly thought when signing up for the class "I want to be able to speak this language"

So, the moral of the story: SPEAK UP! BE HEARD! OTHER THINGS WRITTEN IN CAPITAL LETTERS!!!