German Grammar- please help!

If you are talking about attending a specific school form or a specific school (by name), the least ambiguous way would be: "gehen auf+accusative" (literally: 'to go onto the xy school'). Or you say "besuchen" with direct object (accusative case).

"My brother had already gone to the grammar school"
Translations (past perfect tense / pluperfect):

A) Mein Bruder war schon [auf {die Grammar School}ᴬᴷᴷ] gegangen.
B) Mein Bruder hatte schon [die Grammar School]ᴬᴷᴷ besucht.

However, just from the sentence alone, I can't tell which one would fit better in the context of your story, or if a different way of phrasing would be better. So, if you could briefly explain what exactly you meant to say in relation to your story, it may help avoid an error.

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PS: View this post in a web browser, not in The 'studentroom' app as it has issues displaying text with the brackets I use to mark sentence parts and cases.

Thank you for the help. I wanted to say how I wanted to go to the Hauptschule but my brother was already at the grammar school, so I had to go there instead. Does this help?

Ok, that clears it up. The term "Hauptschule" is probably wrong here – if you mean "comprehensive school", it's "Gesamtschule":

I wanted to go to comprehensive school, but because my brother was already at the grammar school, I too had to go there.

All in the simple past tense (a.k.a. the preterite / Präteritum):

=> Ich wollte eigentlich auf eine Gesamtschule gehen,
a) aber weil mein Bruder schon auf der Grammar School war, musste auch ich dorthin gehen.
b) aber weil mein Bruder schon die Grammar School besuchte, musste auch ich dorthin gehen.
c) aber weil mein Bruder schon zur Grammar School ging, musste auch ich dorthin gehen.

The adverb "schon" means "already".
The adverb "eigentlich" means "actually" as in "it was the original plan", but you could also say "ursprünglich" = "originally" here.

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I don't know the exact GCSE syllabus, so if anything here is unknown or unclear to you, just ask.