My friend told me (male, 16 years old): Two girls at my school are a couple. When they have sex with each other: Do they still stay virgins all their lives?

No, they don‘t.
For many, “losing your virginity” means:
Having sex with another person for the first time.
It does not matter what gender the other person is.

“Having sex” does not only mean:
Inserting the penis into the vagina.
A person can also have sex with their mouths or with their hands.
What is important is what the person likes.
It doesn’t matter whether the other person is a man or a woman.

That means:
These girls also lose their virginity.
Men can also lose their virginity.
For many people, this is a very important thing.

For many, it means: Becoming an adult.
But you also become an adult if you don’t have sex.
And not everybody who has already had sex is also an adult because of it.