I've been thinking about this for a long time, so please hear me out. Now, assuming CC never happens, I have a theory about how the CT characters could have saved Schala.
First, Epoch needs to be safe. After the people go to their respective time periods, Crono can use the Epoch to go find Magus (probably in 12000 BC). Once he hooks up with Magus, they can go over to the End of Time, and get another Time Egg from Gaspar. What they'd then need to do is go to 1999 AD, and stop themselves from defeating Lavos, to ensure that Lavos does indeed destroy the world. Once they accomplish that, Crono and co. could find Nordstein Bekkler and get a Schala clone (they can ask Gaspar for his location). Then, they can climb Death Peak again and revive Schala. After all, she was extremely important in the history of the planet.
If you guys see anything blatently wrong with this, let me know. My ideas have been crushed before, so I'm not scared or anything!