"The shamash should be placed a little higher than the other lights, in order that it may be obvious that it is not one of the required number of candles. (Kitzur Shulchan Aruch 139:14, Maharil)

 QUESTION:
Where is there a hint in the Torah that the shamash should be placed higher than the other candles?
ANSWER:
There is an expression i n (Isaiah 6:2) "Serafim omdim mema'al lo." The word "lo" (לו in Hebrew) has the numerical value of 36, which is the total number of candles lit during the eight days of Chanukah.
"Serafim" means "the fiery ones," a reference "that the shamashim that kindle the candles — "mema'al" — should be placed above — "lo" — the 36 candles. (מהרי״ל) 

Vedibarta Bam on Chanukah