Which is better Although it affected my grade OR Although it effected my grade?

The correct usage is "although it affected my grade".

Affect vs. effect is one of the more difficult distinctions in English. Both can be either verbs or nouns, although affect is more common as a verb and effect is more common as a noun.

"Affect" as a verb means to influence, act on, or change. This is why "although it affected my grade" is correct.

"Effect" as a verb means to bring about, accomplish, or make happen. For example, "The goal of the meeting was to effect a resolution to the dispute." This is why "although it effected my grade" is not correct.