When Chang's paint bomb explodes, the explosions can be clearly seen emanating from either side of the study room door rather than from Chang. Also, Britta is not visible in the shot even though she had been standing upright near Jeff before he ran.
During the scene when Senor Chang enters the library and shoots his gun all over the room, he runs out of Carbon Dioxide in his gun. Later, when Jeff runs into Dean Pelton's office, and shoots it all over the room, it is fine. Between the two scenes is about a minute - where did he get carbon dioxide from?