I gave a secret gift to my favorite student last year but nothing to the other students
Why you don't give the gift to other students?
I want the gifted student to come back this year (and hopefully next year, of course).
You really shouldn't play favorites with students. You should treat others similarly and give them gifts.
It's fine since no one else knows about this.
No, it isn't fine. Playing favorites as a teacher is immoral and unfair to your students.