Does this also mean that she studying in school, as in "she goes to school"?
Yes, it also means that.
Can “abla” be used with strangers (women) as a mean of respect?
It is not popular among educated people. You had better use "hanımefendi" but in spoken it is "hanfendi".
I would say no. I hear "abi" a lot used between younger guys to show respect to each other, but I (personally) have never heard "abla." I sometimes call my closer friends "abla" though :)
Why does it sound like she's saying "bokula"? Is it just me?
I don't hear it at all.
There is liaison here. Ablam okula gidiyor ---> Abla -m'okula gidiyor
"Ablam okula gidiyor."
Translation: My older sister is going to school.
My sister is going to school. - Correct other English answer accepted by Duo.
To be courteous & technically correct I would address an older sister as "abla."