Embarrassment is a common element. Like in the story where a girl was in a public place and had no idea how to deal with her first period. Another is the lack of preparedness. Many girls don't expect it when it first happens and don't have the necessary supplies.
Superstition plays a big role. For example, the belief that a menstruating woman is unclean or brings bad luck. This superstition is used to justify the cruel treatment of women in these stories, which adds to the horror aspect.
Embarrassment is a common element. Like the situation where a girl is in a public place and suddenly realizes her period has started and she has no way to deal with it.
One common element is unpreparedness. Like not having menstrual products when the period starts. Another is embarrassment. For example, getting a stain on clothes in a public place. Also, the fear of others noticing or making a big deal out of it.
One common element is the unexpected start of the period, like when you're wearing white clothes or in a situation where it's very inconvenient to deal with it.
Ghosts are very common. Like the spirits of students or teachers who died on campus. Another element is old, abandoned places in the school such as basements or attics. These places are often seen as spooky and are the settings for many horror stories.
One common element is unfair treatment. For example, teachers might have favorites and give those students better opportunities or grades while being overly critical of others.
One common element is unruly students. They can disrupt the class, be disrespectful, or engage in cheating, which creates a lot of stress for the teacher.
Leakage is a common one. For example, you might be wearing white pants and suddenly notice a stain, which is really embarrassing. Another is the pain and discomfort. Cramps can be so severe that it's hard to move or function normally. Also, being caught without proper supplies like pads or tampons when the period starts suddenly can be a horror story in itself.
Supernatural elements are prevalent. Things like ghosts, vampires, and witches often feature. For example, in stories set in the Middle Ages, witches were a common fear. These supernatural beings bring an element of the unknown and the uncontrollable which is very scary. Also, the social and cultural beliefs of the period play into how these elements are presented. In more religious periods, the horror might be related to damnation or the devil.
A common one is when you have diarrhea during your period. It can be so unexpected and inconvenient, especially if you're not near a toilet. You might be in the middle of something important and then suddenly have to run to the bathroom.