Is it really necesssary for girls to wear panties under their shorts?

I think it is what is most comfortable and beneficial to her vagina.

The seam that connects the two *legs* of the shorts falls directly beneath where her privates are. That seam, rubbing against the skin, causing friction, could create a lot of discomfort, even unto infection.

Necessary? As in, will people somehow see her buttocks or privates? Unlikely, although, someone sitting across from her might be able to see up the leg of the shorts if she is seated, depending how long they are. Personally, I think necessary in order to properly care for her body.

Who wants a seam up the tw@t all day long? Ouchie.

Yeah, it's helpful. Especially on their periods. Even when they are not having period, it's good for hygienic reasons. Vaginal discharge can cause some skin irritation or infection near the vagina I heard. Boys might get excited to find a girl wearing no undergarments. So maybe they will care.

I'd say it was necessary for hygiene reasons.

As for whether or not boys care - it's none of their business!