"I have been playing baseball since middle school."
It depends. The sentence examples I listed above can all be translated using I/He/She/They. Personal pronouns should only be omitted when it is clear from the context who is being referred to.
In the case of Duolingo translations, it is best to answer it using the corresponding personal pronoun or risk being marked incorrect.
Doesnt this imply playing baseball nonstop since middle school age? Wouldn't you want to use "shimasu" to say this is a sport you've been into since middle school?
It means that you have done it regularly (or sporadically) since that time.
The ～ています form is used for continuous actions, still ongoing.
・(いま）やきゅうをしています - I'm playing baseball (now).
・やきゅうをします - I play baseball.
・やきゅうをしていました - I was playing baseball.
・(むかし）よくやきゅうをしていました - I used to play baseball often (a long time ago).
・(だいがくで）やきゅうをしました - I played baseball in college.
Why is the "わたしは" required? I've never seen it required previously...surely "I" would be implied without further context?
Is there much of a difference between 中学 and 中学校? FWIW I tried about 20 different ways of answering this and it wouldn't accept any of them until I used the suggested answer.
The full word is 中等学校. High school is 高等学校.
Normal colloquial abbreviations of the above are 中学 and 高校.
However, 中学校 is the semi-formal and default equivalent of 中等学校, which is actually not seen so often.
So, while a middle or high-school's proper name might include 中学校 or 高等学校, conversationally you should stick with 中学 and 高校.