As it is used in the passage above, the meaning of told on is "reported."
In this passage, told means to be reported. This passage is the typical kids going to the principal's office scenario. However, from the first sentence we know that the narrator believes Niu Fen or Zhang Wei reported the narrator and his accomplice(s) to the leaders. Since they were called to the office, it is unlikely anything was concealed, and the word understood simply does not make sense.
