Which is correct ? Quick!! 急!!

Both are in correct!

英文介紹自己可以有幾種說法，但你列的都可能係用中文思想構成的英文字...

列出一種最直接簡單的講/寫法：
[假設S1意思是中一吧]

I am XXX, a S1 student.
這是先介紹自己的姓名...然後身份(由於你不是唯一或獨一的中一學生...所以邏輯上你只能說你是一個中一學生，不會是the S1 student.除非你講的身份是唯一的，例如你是學生會的會長: I am XXX, the chair of the student union.因為在你講述的學校範圍內只有一個學生會會長這職位，如果講的對象是全個地區/城市/世界...則要加上..., the chair of the student union of YYY School/College/Univeristy.等附加資料才能完整。

I am S1 student XXX.

因為在'I am the student of S1' 中, 我們不應用 THE student 因為全S1吾系只得一個student. 正確應用A student. the通常是特別稱呼(像名字etc), 或只有一個的事物才適用

I am S1 student XXX. I am Yahoo+ member 匿名.

I am a student of S1. I am a member of Yahoo+.

l am "a" S1 student !

"I am the student of S1" no grammatical mistakes but meaningless.....

I am S1 student XXX. & I am the student of S1.
No grammatical mistakes were found either, they are readable and acceptable.