[Coda]

First sentence is grammatically correct, but I'm uncertain how something can be "excessively" boring -- it seems to me that if it's boring it's boring, and there's no amount of boredom that isn't more than you want. ("not just boring, but TOO boring")

Fourth sentence: You probably wanted "kedo" instead of "kara" there (or maybe "ga", for a less cause-and-effect form of "but") -- what you said is "I have no M&M's BECAUSE I'm in economics class." Also, you use "ni", not "de", when describing where something/someone is. (What you said could possibly be interpreted as "I exist as an economics class.") And to answer your parenthetical, the verb you're looking for is 切らす (kirasu, "to be out of; to run out; to exhaust") -- so perhaps 経済の授業にいるが、もうM＆Msが切らしちゃった。("I'm in economics class, but I'm (regretfully) already out of M&Ms." -- if you need explanation on -chatta, just ask.)

I'm not so sure about "sou ne" there. My dictionary agrees that it means "oh well" but I've always heard it said to mean "that's right." Perhaps you want "shou ga nai" (or "shikata nai") meaning literally "there's nothing that can be done about it".

As for "I won't use it again", I'm not certain... Maybe "zenzen tsukawanai tsumori desu" ("I don't intend to ever use it.") -- Google Translate at least seems to indicate this means "I will not use at all". EDIT: Wait, the negative is in the wrong place; that means "I intend to never use it." Perhaps "tsukau tsumori ja nai" -- "I have no intent to use it."

"benkyou shinakereba naranai" is unusually formal considering the casual tone you've been using in the rest of the post. A more casual, slang way to say it would just be "benkyou shinakya". (My professor said "benkyou shinakucha" is also acceptable but I've not actually heard it.)