So the plural forms of ser are
Nosotros somos
Vosotros sois
ellos/ellas son
I assume that your teacher means for you to do sentences like
Los chicos son guapos. (for the boys)
and
Las chicas son guapas. (for the girls)
If you are including yourself in either of these groups you would do something like
Las chicas y yo somos muy simpáticas. (if you are a girl)
or
Los chicos y yo somos muy simpáticos. (if you are a boy)
If you are talking to the group of boys or girls directly I assume you would do something like
Vosotros sois muy tontos. (for the boys)
or
Vosotros sois muy tontas. (for the girls)
I hope this helps!
