"The Tell-Tale Heart" is a great story to read in schools for several reasons. One is that it is relatively short, so it holds the attention of students throughout the entire thing. If something gets too long, students lose focus. Also, it is very suspenseful. From the very beginning the narrator confesses to a murder, and presents an intriguing case of potential insanity. Students love tales about insane murderers; just look at the popularity of the horror/slasher movies in recent years. About 50 of them come out every month. :) So, it has a very intriguing and interesting storyline that grabs you from the very beginning. Another reason that it is used in schools is that it is readable; the language isn't too difficult, the plot-line is easy to understand, the narrator explains what happens very clearly, and it has a super cool ending. Also, it teaches some great lessons about the power of guilt, and the power of the conscience. The beating heart, symbolizing the narrator's guilt, is a great lesson that one usually can't do bad deeds without some negative repercussions in the guilt department. It also provides great debates on the question of whether the narrator is indeed insane or not, so great class discussions come out of it.
All of those reasons provide adequate reason for schools to read it in their classes. As for why Poe wrote it, Poe was quite a dark character himself; he had suffered a lot of tragedy in his own life, so was acquainted with the darker parts of life, and of the soul. Plus, he was a very entertaining storyteller, who liked to present stories from the perspectives of the bad guys; many other stories of his tell of a murder or bad deed done, from the point of view of the murderer. He was a part of the Americah Gothic movement of writing, which liked to focus not on mankind's ideal traits, but on mankind's darker side-their potential for evil and vice. So, being a part of that gothic movement, his tales tend to have that twist to them.
It sure is a fun tale to read and discuss; I hope that you enjoyed it! I also provided some links below that discuss the themes and historical background, and that might help too.