Headphone leakage

I had the same exact problem, but I didn't want to go with IEM's, as I have waxy ears.  I picked my Beyerdynamic DT770 Pro-80 headphones, $200-ish, and got the genuine leather ear pads from the beyerdynamic website.  The leather pads isolate sound better than the stock velour pads of the DT770's.  However, they are kinda a pain to carry around, as they don't fold up at all.  They also might require an amplifier, if you want to drive them really loud, which an ipod cannot do itself.

You'll likely want a preferably circumaural, closed headphone with synthetic leather or leather ear pads.  These are generally big headphones, so you might need an amplifier, especially at a higher price point.  Here are some ideas:

$80-$100 - Sony XB700.  Isolate well with the monster truck tire ear cushions, but are only recommended for bassheads.  No amplifier required.

$150 - Audio-Technica ATH-M50.  Isolate decently, better than beats, and have good sound quality, somewhat bassy but nothing like Beats or the Sony XB series.  No amplifier required.

$160 - Sennheiser HD380.  Isolate very well, and have a very flat studio monitor sound, not accentuating the bass much.  A good upgrade over the HD280 as far as I've heard, and do not require an amplifier.

$200 - Beyerdynamic DT770 Pro-80 with $50 leather ear pads purchased separately from beyerdynamic's website.  Rather bassy headphones.  Might need an amplifier, I had to turn up my iPod to 95% volume without an amplifier, so it's workable, but not perfect.

$300 - Sony XB1000.  Will need an amplifier, these headphones are HUGE, but they isolate very well with the monster truck tire ear cushions.  Again, designed for the basshead.

$350 - Denon AH-D2000.  Generally well-reviewed here.  I don't think these require an amplifier, but it can't hurt, only help.  Big, fully circumaural cans with good bass and isolation.

There's always more headphones, these are just the ones off the top of my head.

For an amplifier, if you need one, I'd recommend the FiiO E7.  It is a DAC and amplifier, so you can use it with both your ipod and laptop, and charge it with your laptop.  You should also get a Line-Out-Dock (LOD) cable to hook it up to your iPod through the dock.

However, in one of those really quiet, well isolated college classrooms, even my DT770 Pro-80 with leather aviation ear pads would still leak sound noticeably.  Very awkward when you drop Skrillex during a five-minute break in a writing seminar class, not expecting that.  But with a small amount of ambient noise, you can barely hear it from outside, if at all.