Magical 12th Graders

After the death of her older sister, Yeorum Han enters her final year of high school. But when a nap in class runs long, she wakes up to find her biology teacher and best friend in cosplay... fighting monsters?! Faced with death, Yeorum chooses the path of a magical girl, and resolves to find out the truth behind her sister's suicide.

(Source: LINE Webtoon)