"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.
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.
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.
In my experience, we call the junior high school 中学校 in conversation when talking about a school / building / level of study. When we shorten the name of the school (so say there is a junior high school called 西東京中学校 Nishi Tokyo Chuugakkou), we would call it 西中 (nishi chuu). I think what is happening in this sentence is that we are not talking about a specific school, but the time when we were a junior high school student. Saying 中学から is saying "from when I was a junior high school student" and 中学校 doesn't carry that nuance.
小学校 basically should be used for a building of a primary school. For example, あれが小学校の建物です。 小学校に行きます。
Native Japanese speakers don’t say小学に行きます.
小学 should be an abbreviation of小学生. For example, 私の妹は小学2年生です。
The usage of中学 and中学校 is similar to that above.
However, when you say ‘when I was a primary school student,…’, you should not say 小学の時. 小学生の時 would be better. However, when you say ‘when I was a junior high school student,…’, you can say either中学の時 or 中学生の時.
する is a verb which in this case means "play". The の makes the verb into the noun "playing".
野球が好きです。 (yakyuu ga suki desu)
I like baseball.
野球をするのが好きです。 (yakyuu o suru no ga suki desu)
I like playing baseball.
In this sentence, the word "playing" is part of the present continuous, meaning that it is an action that started in the past and is still going on now. The present continuous is formed by the te-form of the verb + います.
野球をします。 (yakyuu o shimasu)
I play baseball.
野球をしています。 (yakyuu o shite imasu)
I am playing baseball. / I have been playing baseball.