This isn't exactly a homework question, but it's something I've seen in books recently, and it's bothering me. The way I've been taught, it's grammatically incorrect, and my teacher agrees; however, she says she's been seeing it in a lot of recent books as well.

This sentence:
Lonnie looked up, "Really?"

That comma....? Is that correct to have it there? It should be a period, right? Or is this some new way of writing that's been adopted?

To me, it looks like careless writing or lack of good editing. I agree that there should be a period there instead of a comma. Commas are used for this purpose when there is some kind of "reporting" verb, such as said, asked, wondered, and so on.

http://www.google.com/search?q=commas+and+direct+address&oq=commas+and+direct+address&aqs=chrome..69i57j0l5.8042j0j4&sourceid=chrome&espv=210&es_sm=119&ie=UTF-8#es_sm=119&espv=210&q=commas+and+direct+speech

One thing to keep in mind is that today's proliferation of video and audio and live "journalism" by just about anyone who can hold a camera or who has a device with a keyboard is influencing language, including pronunciation, spelling, punctuation, and even meaning. Here's an interesting blog post about how some fairly common words have changed over time ... and why!
http://mentalfloss.com/article/49022/7-words-came-about-people-getting-them-wrong

One of my pet peeves is the lack of the serial comma (aka Oxford comma) which often affects meaning.
http://mentalfloss.com/search?term=oxford+comma

Wow. I never would've thought that things had just been influenced by people, but it does make sense. I've been saying those words wrong all my life, because that's how I've heard them since I was little!

The Oxford comma irritates me, too, if it's missing. My teacher says that that's also something people are just taught to do differently, so she doesn't count it wrong against those who do it like that.

Thanks for all of the input; I really learned a lot more than I was expecting to! I am to be a writer when I get older, so I try to stay on top of all of these writing things. Still, I worry that, by seeing this stuff so often, it'll just kind of rub off on my writing. I haven't had a problem so far...

The sentence you provided, "Lonnie looked up, 'Really?'," does use a comma within the quotation marks.

In traditional grammar rules, it is generally correct to use a period instead of a comma to separate the dialogue from the attribution, like this:

Lonnie looked up. "Really?"

The use of a comma before the closing quotation mark, as seen in your sentence, is more commonly known as a "comma splice." Traditionally, a comma splice is considered to be a grammatical error because it joins two independent clauses (or complete sentences) without the appropriate punctuation or conjunction.

However, in recent years, some English style guides and publishing houses have adopted a more flexible approach when it comes to using comma splices in dialogue. This is often seen in contemporary literature, where authors may choose to prioritize a certain writing style or rhythm over strict adherence to traditional grammar rules.

While comma splices in dialogue are still considered nonstandard in formal writing, they have become more accepted and commonplace in certain contexts or genres. Consequently, you may encounter them more frequently in books, magazines, or online articles that embrace a more casual or conversational writing style.

So, to answer your question, both versions of the sentence (with a comma or a period) can be considered correct, depending on the context and writing style being used. It ultimately comes down to the author's preference and the conventions they choose to follow.