Pick your battles. If you do it in a room with a bunch of people who are probably all against you like in a class room you will just be shouted over and you can’t possible defend against hundreds of remarks at once. Try it out in smaller groups when you’re chatting with friends.
And be prepared to have the feeling afterwards of, ” I shoulda said this and I shoulda mentioned that”. It’s all normal but you get better in time.
Another way to interject in a classroom (which is sort of dishonest but I’ve done it before) is phrase your disapproval of what they are saying as a question. Something like.. ” A friend of mine once told me such and such about economies (or whatever) and I didn’t know how to answer him.”
Then sit back and listen to what they say. Load that reply into you mind and go do research on it and find out whats wrong with it either factually or logically.
That is assuming it is indeed wrong. One must always be willing to question even their deepest of convictions and honest enough to admit when something is right.