Mathematics is listed as a class in Mal's Diary when she is writing her schedule. Not withstanding Remedial Goodness 101, Mathematics is Mal's fourth class. In Mal's Spellbook It is revealed that both Jay and Evie have it as their seventh class when Jay wrote his schedule and Evie mentioned that with the exception of Dragon Anatomy (Jay's first period class) and and Basic Chivalry (Jay's fifth period), their classes. While which number class this is for Carlos is unknown and not mentioned considering that Mathematics is important in any school and the fact that all three of the other VKs take it, the class is a requirement. All that can be known is that it is not Carlos' fifth period (once again not withstanding Remedial Goodness 101).