Hi !!
The only correct sentence is = tu es sympa
les garçons est petites = les garçons sont petits (the boys are small verb = 3rd person plural "être", petits= adjective masculine plural for boys_ petites is feminine plural)
la fille est seize = la fille a seize ans (the girl is sixteen )
la classe est intéressant = la classe est intéressante ("la classe" is a feminine noun so, the adjective has to be feminine)
hope I helped !!