Both youngsters were due to be out of contract at the end of the season but have been rewarded for their impressive performances for the U20 and development squads.
Striker Atajic has contributed 11 league goals this season and scored with a sublime overhead kick during the Youth Cup campaign, while Fisher has proved a consistent performer at right-back.
Development coach Stevie Frail said: "They have been in good form this season and they deserve this.
"We have been impressed with them but the work gets even harder now.
"They are striving to get into the first team but they must continue to train hard and hopefully their development will continue as it has done in the last few months."