Disabling laptop monitor on boot?

Disabling laptop monitor on boot?

I'm using a laptop with Ubuntu (no graphical desktop) to do all of my
work. The resolution is awful, at 1024x576 or something similar. So I just
close my laptop lid and plug it into an external monitor - but the
external one never seems to stretch out to its full size.
The reason for this is that the laptop monitor is always enabled - even
when the laptop lid is closed. As a result, the external monitor never
assumes the full size, and any time I set GRUB to use a resolution higher
than 1024x576, the external screen ignores it.
Is there any way to disable the laptop screen in GRUB, or to specify which
screen I want as the primary one? I've tried Google, and absolutely
nothing comes up.
(The computer is a Lenovo S10e, if it helps at all.)