Another translation can be "Byl jsi tam s učiteli." "You were" can be both "byl jsi" (singular) and "byli jste" (plural).
I wrote: Byl jsi tam s ty uceteli, and DL rejected it. Why?
Try to look up the correct form of the demonstrative pronoun. The nominative is "ti učitelé", the genitive would be "těch učitelů", the accusative is "ty učitele", etc., and here you need the instrumental case.