Some information

Operating system: Windows 10
Java version: 1.8.0_202
Minecraft version: 1.12.2
Modpack: FTB Academy Minecraft 1.12.2 (v1.1.0)
Baritone version: 1.2.15 (also applies to 1.2.14, other versions not tested)
Forge mods (if used): See "Mod List" here

Exception, error or logs

Crashlog with VanillaFix enabled
Crashlog from FTBLauncher with Vanillafix enabled

How to reproduce

Download FTB Academy (v1.1.0, newer versions untested)
Install Baritone
Crash
Disable VanillaFix with "Edit mods"
Not crashing

Disable vanillafix then, baritone is obviously incompatible with it.

The other option (which rarely works) is to try renaming baritone to !baritone, this should make it load the mixins before other mods.