While I'm a high school student my parents pay my tuition fees but when I become a university student, I'll have to pay my fees myself.
「大学生になると」= When I (youなど)become a university student...
「高校生のときは親に学費を払ってもらってたが、大学生になると自分で払わなければならない」= While I'm a high school student my parents pay my tuition fees but when I become a university student, I'll have to pay my fees myself.
ボキャブラリー
while = ~の間
high school student = 高校生
my parents = 自分の親
pay = 払う
tuition fees = 学費
but = だけど
when I become = ~になると
a university student = 大学生
have to = しなければならない
myself = 自分で
例文:"My parents paid my tuition for me while I was a high school student, but when I become a college student I will have to pay my own tuition."=「高校生のときは親に学費を払ってもらってたが、大学生になると自分で払わなければならない。」
例文:"My parents paid my tuition for me while I was a high school student, but after I become a college student I will have to pay my own tuition."=「高校生のときは親に学費を払ってもらってたが、大学生になると自分で払わなければならない。」
例文:"My parents paid my tuition for me while I was a high school student, but once I become a college student I will have to pay my own tuition."=「高校生のときは親に学費を払ってもらってたが、大学生になると自分で払わなければならない。」
*アメリカ英語だと、"university student"も言えますが、アメリカの小さい大学は"university"と言えないので、"college student"はお勧めです。