Jens Keller's side, who host Real next Wednesday in their Champions League last 16 first leg, could have moved second with a very handsome win but instead stay fourth after struggling to get going against an enterprising Mainz outfit.
The draw ended Schalke's run of four wins in a row since the mid-season break.
Jefferson Farfan hit the bar direct from a corner and fired just wide with two other efforts but the best chance of the game arguably fell to Mainz's Japanese striker Shinji Okazaki, who forced a smart save from Ralf Faehrmann.
Mainz, who remain seventh, had a couple of other decent opportunities and two half shouts for a penalty.
Second-placed Bayer Leverkusen (43 points) and Borussia Dortmund in third (42) can extend their advantages over Schalke (41) on Saturday at Wolfsburg and beleaguered Hamburg respectively.
Defending champions Bayern Munich, who have a 16-point lead at the top, travel to Hannover on Sunday.