[cranks]

Dear OP,

私(わたし)は土曜日（どようび）に時々（ときどき）学� ��（がっこう）に行きます。
私は時々土曜日に学校に行きます。

are both fine. I'm not very good at explaining grammar, but as far as I can think of, there is no rule in the order you write "time" and "place". Japanese word order is pretty flexible in general. For instance,

彼(かれ)がそこに行（い）く
そこに彼が行く

They both mean the same thing, "He goes there", and it is not even an inversion. Neither is "There he goes", although the latter has a hint of emphasis on "there".

As for 「たまに」, it's just a slightly more informal form of 時々. I would drop "私は" before changing 時々 to たまに if I wanted to sound more colloquial though. 時々土曜日に学校にいく(んだけど、) or 土曜日に時々学校にいく(んだけど、) sounds more speech-like.

Hope this helps.

P.S.
There are many different rules in using は and が, and it is highly dependent on the context. There are even some books dedicated to the topic, and it's best you consult a good grammar book, but your examples here sound correct to me.