This was brought up recently, so let's see how we do with it.

Fumizuki Academy prides itself on it's unique testing system where by your class is based on your testing score, A being the highest and get the best equipment where as F being the lowest get all the crappy equipment.
Using an avatar based Augmented Reality, the students can summon them to battle with the stakes often being better equipment, how strong your avatar is, is based on your test score also.
Himeji falls ill on the day of the placement test, forcing her to join Class F and their unique set of students, Akihisa feeling bad for Himeji, wants her to be re-tested as she's actually a genius. Sensing a chance to takedown the system deeming it unfair to the lower classes, class rep Yuuji sets the students on a quest to use the summoner test war to bring down the whole school proving that you shouldn't underestimate the lower classes.
This is a very anime way of explaining that test scores don't mean everything, something that a number of school systems need to remember. It's definitely the characters that make this series work, Yuuji is more intelligent than he let's on as his real reason for going to class F was to avoid a stalker, Akihisa clearly has learning difficulties as his avatar can do things others can't, Minami is German and can't read Kanji, Himeji herself maybe book smart but is socially bankrupt, then you have Hideyoshi with the running gag that no one can tell his gender (he's male but looks more feminine than his twin sister)
It's a delightful cast of characters that really make the anime work, the plot is a little too much and way more anime than most.
Dub is fine and features some great moments.
Final Verdict: A great cast of characters in a school series that goes way too anime at times.