Midfielder Wallace, 19, impressed on-loan at Stockport County and was handed the captain's armband towards the end of his stint.
"I am delighted to sign a new deal at the Club," Wallace told evertonfc.com. "I am made up it is sorted and I am just looking forward to pre-season."
Nsiala, a central defender who can also operate at right-back, is another youngster who spent time in League Two last season - with Stockport's Cheshire neighbours Macclesfield.
"I am happy, and dead excited - I'm happy that I have been given another challenge," he told evertonfc.com. "Getting on the bench and scoring while on loan have been ups for me this season and I am looking forward to next season."