You can skip grade. But the practical exams are held once a
year. So got to have more than 2 months of commitment. Also, its
not very much about getting the certs, everyone out there has it
lol. It has become more of a invaluable piece of paper. What is
good is you truly learn what music and piano has to offer. There
are many Grade 8s out there who can't play very well.
The sooner you get your cert, the better it is, quality of certs
are going down. lol. 
TYING you're a piano teacher too? 
I did Grade 1, 3, 5 and 8 with ABRSM, and 2, 4, 6 with
Guildhall, skipped Grade 7. Now on LRSM. Finished DipABRSM already.
Piano performance, not the other two 
Oh btw TS, please look at your PM.