I think 我妹妹十一点上学 should be accepted since you usually don't need to include 的 when referring to people you are close to.

You don't need 的 for family members. Please fix all of your sentences.

Still not fixed. :(

I reported the mistake. We don't need "de" for familiars.

I keep reporting this too.

Agree with this one,still typo

To my understanding, isn't there usually a phrase to articulate whether the 11 is a.m. or p.m.? Are there circumstances where, like here, it is omitted?