Ranma has always shown to have problems with his pride as shown any time he loses. However it is only when he loses to someone with an unfair advantage. When he loses to someone like Herb, Cologne, or Happosai he just become more determined to get better, you only see him freak out during the Seal of the Battle God, Crybaby Glasses, Super Soba, and the Battle Dogi arcs as those loses step on his pride as a martial artist. It for this reason why it so understandable why being called a monster is such a huge berserk button for him.
In the manga at least Ranma was always shown to be very intelligent the only problem is that he didnt take school seriously and refused to conform only doing just enough so that he doesnt get back grades. Not only is Ranma shown to be well above the ranking of those required to take make up classes, but the only reason he was expelled in the first place was the Saffron incident forced him to take off for quite awhile which was because he only did the minimal amount of work required not to get bad grades.