Translation:I have been playing baseball since middle school.
私は中学から野球をしています。I guess している can mean “is doing” as well as “has been doing”? Obviously the latter in this context.
Yes. Because the sentence has "から" which shows a starting point in the past and "しています" which shows that the person is still doing it now, we would use "have been."
(I am/she is/they are) studying.
(I have/she has/they have) been studying Japanese since July.
(I am/she is/they are) watching.
(I have/she has/they have) been watching TV since 8:00.
As a new learner, to me this sentence means the speaker started a baseball game in middle school, they have been playing that game constantly, they are still playing, and never stopped because of the している ending.
Japanese doesn't have a perfect tense, so you're right, using the present progressive here sounds strange to us. If it was an actual game that this person was playing for years and years, they would have specified 試合 (shiai, game).
Japanese doesn't have a perfect tense. ～てある (~te aru) is a form of verbs that implies that someone has done something for a purpose, which has no direct English translation.
When we use words like "since", we have to use the perfect form of the verb (have + past participle), which is why "I have been playing" is correct.