The grammatically correct sentence is Tu es sympa.
The correct forms of the other sentences are as follows: Les garçons sont petits. (in the original sentence, petites is feminine. Garçons is masculine; they do not go together) La fille a seize ans. (the French do not say "she is sixteen" but "she has sixteen years") J’ai une classe intéressante. (fem/masc agreement, once again)
