I am writing a series of mini-novels based on a group of teenagers who all have different backgrounds and ethnicities. How should I introduce each character? I mean, they are all kind of main characters. Each of them is going to have a major issue in each book that will be dealt with. In a way, these are books to help teenagers. Are there similar books to mine that I can look to for reference? Is it best to have one main character that tells the story? What do you think I should do?
If you are providing character diversity in part to make it easier for a diverse readership to identify with at least one character, then you probably do not want one character as the narrator of the entire book. A reader naturally feels a greater emotional connection to such a narrator. Furthermore, such makes it more difficult to express the less externally observable traits of the characters (thoughts and feelings).
So, if that is your goal (as seems to be the case based on each character having an issue to deal with), you should probably use a third person narrator, different characters as narrators for different portions, or some combination. With the stated length ("mini-novel") and target audience, changing the narrator sufficiently often to give each significant character equal play may be difficult.
With respect to introducing multiple main characters, there are many options. The book might start out in a scene with most or all of these characters in a common situation (e.g., having lunch together at the school cafeteria, being in line to sign up for something [possibly with one or more characters behind the sign-up table, signing up for something else, or just hanging around in the area], or being addressed by the non-main character leader of a homecoming float-building group), very briefly describing each character. (Third person narrative works well in such a setting.) The action might then focus on one or two of the main characters with the focus changing over longer periods of time (with gradual or abrupt transitions). Or there might be interaction among all or most of the main characters right from the start--e.g., friendly conversation, intentional rudeness to exclude an outsider, some characters gossiping about others.
It is also possible to gradually introduce characters, starting with two or three in the first scene and linking each of these to the other characters. This would tend to fit well with using multiple narrators. The introduction order can be used to contrast the significance of the character or to emphasize the relative isolation of the characters introduced later. E.g., quiet beret girl, who is not introduced until half way through the book, might be key to half the characters understanding their issues or she might be a character of no special importance whose issue is loneliness or telling a friend something difficult (both play on the isolation--in the first case being descriptive of her current state, in the latter case the isolation is something that she fears may come if she tells her friend). If quiet beret girl has the former role, it may be appropriate to foreshadow that she is significant by giving her one or two cameos early in the story. For example, the following both introduces and excludes beret girl:
"Hey, Susan," he called from behind the table.
"Hi, Juan," she smiled, leaning to see past a girl in a grey beret.
When introducing characters, particularly for your audience, length, and character count, it is helpful to present early a few memorable points onto which the reader can latch. E.g., quiet beret girl, non-nerd with glasses guy, rough-and-tumble string bean guy, brainy fashionable girl, poor leader girl, totally average guy. Such helps the reader establish a base upon which to build an understanding of the character and more easily identify the character in later scenes.
The "Power of Five" series by Anthony Horowitz tells the story of a group of five characters, and each book has one other of these characters as the protagonist (and tells the story from his or her perspective). Sharon Shinn in her "Twelve Houses" series does the same.
Both start the first book like a normal novel about one character. In the course of the story the other characters are introduced (the first protagonist meets them). Each book tells a complete adventure and can be read on its own. The protagonist of that adventure is the protagonist of the book, the other group members are supporting characters. At the same time, a larger story arc runs in the background. This background story culminates in the last volume. The whole group are the heroes of that background story, and each individual's adventure is a key stone in the large storyline.
Both these series are constructed well, I can recommend them as good examples for this type of storytelling. (They have other weaknesses that are not relevant to your question. The best individual volumes from a pure reader perspective in my opinion are the first by Horowitz, "Raven's Gate", and a post-series supplemental volume by Shinn, "Fortune and Fate".)
OK, this is not great literature -- possibly not even good literature -- but "The Goddess Girls" series of books has many of the features you are describing. Some tween girls devour these books. And if you look up that series at a bookstore or library, you are bound to be directed to similar series, which will direct you to similar series, which will ....