I am starting a new sleep schedule and I will have to sleep on the
bus/at work/at school, etc. various places. I have devised a plan
where I would be able to use an alarm clock without disturbing
anybody, and without being disturbed by anyone other than the alarm
clock. I need an alarm clock that provides output to headphones, and I
need those headphones to be with built-in earplugs ( i.e. only the
output from the headphones is heard, and if there's no output there,
nothing is heard ).

If you're adventurous, it's not so hard to get a mono 1/8" jack such
as http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&product%5Fid=274-251
and place it in line with any standard radio alarm clock's speaker.
With a cheap $9-10 alarm, bore a hole to fit the jack somewhere near
the speaker. Open the box, clip the leads to the speaker, remove some
insulation on the leads and solder them (or merely wrap them about)
one each of the two connectors of the jack. The alarm is forever
silent and waiting patiently for your
http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&product%5Fid=33-180
earphone.

Caveat: please don't do this with the alarm clock plugged in :=) and
also the alarm clock must have a SPEAKER. Travel alarm clocks with the
piezo buzzer/whistle will not work. Radio alarm clocks are optimal
when set to "Wake to radio" mode.