tekkit classic does use forge only a earlier version the bug is only with forge #506-517 and since tekkit lite 0.5.7 uses forge 517 it conflicts with optifine
here is the post on optifine's thread
Preview OptiFine HD D3 Ultra for Minecraft 1.4.6 and 1.4.7 (added option Custom Sky based on MCPatcher's Better Skies, fixed CustomSky negative rotation axis, fixed TwilightForest maps, compatible with Forge #518, not compatible with Forge #506-516)
it really simple just update your forge to 518 and its fixed