If you have the admin account you should be able to change the required settings.
The lock screen is affected by 3 settings.
The first is the System Preferences -> security & privacy -> general -> Require password 'some time' after sleep or screen saver begins.
When you try to change this you should get a prompt for a password. If this does not work you may not have the admin account.
This setting dictates how long after the computer sleeps till it locks.
The second setting setting is how long it takes to sleep in the first place.
System Preferences -> energy saver -> battery and power adaptor -> turn display off after 'some time'
This would make the computer wait longer to turn off the screen.
System Preferences -> desktop and screen saver -> start after 'some time'.
This is the screen saver setting as the screen locks after the screen saver or display sleeping.
If you don't have the admin account and aren't allowed to change these settings then you have to talk to whoever has the admin account.