Help is required from the Natives.

The following idiom is not understandable to me clearly.
To keep you on your toes.

I'm giving a couple of example sentences for correction and advice.
1. My teacher asked the monitor to watch the pupils to keep them on their toes before he'd left the class for a short while.
2. I buy my son a choclate everyday to keep him on his toes in his studies.

Kindly correct me with explanation if you find any mistakes from beginning to the end.Besides,clarify the meaning of the first sentence.

"To keep someone on their toes" means to keep them aware, ready, or paying attention. A math teacher may throw in a hard math problem on an exam to keep his/her students on their toes.

A monitor may watch students during a test so that they won't have the urge to cheat, thus keeping them on their toes.

This is a good online idiom dictionary.

http://dictionary.cambridge.org/default.asp?dict=I

Just type in the main word of the idiom and press search. For example, if you type "toes" in and then click on "keep sb on their toes" ("sb" means "somebody") you'll find that mjd was right.

By the way you should have written "before he left the class" not "before he'd left the class". Use the simple present in a prepositional phrase.

The definition given in that dictionary is:

keep sb on their toes

to force someone to continue giving all their attention and energy to what they are doing