The best verb is [習]{なら}う. Check the dictionary on jisho.org:
to take lessons in; to be taught; to learn (from a teacher); to study (under a teacher); to get training in
The point of the verb 習う is that you have a teacher.
So the whole sentence is:
[私]{わたし}は[高校]{こうこう}のときギターを[習]{なら}っていました。
Watashi wa kōkō no toki gitā o naratte imashita.
Using the verb [習]{なら}う, it is difficult to hide "guitar" and just say "took some lessons." In this case, the noun [習]{なら}い[事]{ごと} is suitable. This means "(taking) lessons".
[私]{わたし}は[高校]{こうこう}のとき[習]{なら}い[事]{ごと}をしていました。
Watashi wa kōkō no toki naraigoto o shite imashita.
Side Note: in some cases you can use [取]{と}る to mean "take (classes)". It has the nuance of choosing from many classes and used often in universities where students can choose what classes they take.