Congratulations on passing your test, and welcome to Pprune!
As mentioned, you have to have the bit of paper in your hand.
The reason? Although the flying school/examiner/you will have checked all the paperwork before submitting it, there are an amazing number of applications that get rejected at first application because something is wrong! (issues include exams out of validity, medicals, wrong signatures, forgetting certified id, cheques bouncing, etc)
As a courtesy, we waive all instructor fees for solo flight while students are awaiting their "bit of paper". So they can go solo, but they can't take a passenger for a few weeks.
Might be worth asking if your school does the same?
And come and visit us sometime.