With that reasoning, you could also decide that classes shouldn't be mixed-gender, because boys might bully girl or girls might bully boys, and it's better to keep everyone in a group of people exactly like them.
Educators are finding that mixing genders actually puts a lot of pressure on boys to behave like girls, ie less excited etc so when they act out, they are medicated or sent to the principals office. Separating boys from girls in the classroom might help boys when they are younger.
And girls benefit from being separated especially during puberty. My friend just sent her daughter to an all-girls middle school that specifically targets girls this age so that they can learn to be more confident during the most awkward age of their lives.
Especially with girls it’s important for kids to learn self esteem and agency, and they can’t do that when bigger, stronger, smarter and faster older kids are dominating everything in the classroom.