A few things to type in if your playing online or even just at school:

/cl_maxpackets 100 ==>
Higher maxpackets increase smoothness but heighten ping. Use this one for school servers

/rate 25000 ==>
Be careful with rate, anything lower than 10000 will get you the boot on many online servers

/com_maxfps 43 ==>
Set your max frame rate, 43 and 76 are considered best bets.

/cl_packetdup 0 ==>
Set the number of duplicate packets sent to the server

/cl_timenudge -25 ==>
Set the time period for snapshots to be delayed

Type these in the console by clicking ~. You will also find a dramatic decrease in lag, especially on online servers.