I want to say:
You still have a long way to go, wish you have a memorable secondary school career
And here's my translation to Chinese:
I'm wondering if it is correct to use 來日方長 here?
It's based on your intention.
来日方长 usually used for the two situations, if you want to tell sb that
"来日方长" is similar to "take your time""no hurry". And we do not use "来日方长" a lot in spoken Chinese.