Where are you located? That will help people to make suggestions.

That is difficult. I have problems finding knee-length dresses appropriate for work and I'm willing to pay whatever is necessary, but there is just not alot out there. I click on "Work Dresses" links on the internet and a majority of the dresses show too much thigh. Skirts may be an easier to find option. I would recommend Sears, JCPenney, Macy's ... department stores will have a better selection & prices.

Having to have the knee covered while sitting definitely makes it more of a challenge.

If you wear leggings underneath, can you wear a little shorter dresses?

I live in West Virginia - which is why I do most of my shopping online.

I double checked the handbook, and the knee doesn't have to be completely covered, so they can be to the top of the knee when sitting. Most of the 'knee length' dresses I've found aren't really knee length!

Also - I have skirts, which is what I wear to class most of the time, but I need some simple dresses for slightly more formal occasions.

For the cheaper dresses you've found - are you referring to strappy dresses? If the length is okay, strappy dresses can be layered in all sorts of ways: with cardigans or shrugs over, tees underneath, etc.

Strappy dresses are allowed - the only problem that I have is the length!

I can't browse through the pics at the moment, but I think at http://www.academichic.com there are several pics of layered dresses or skirts under dresses that would allow you to have the additional length with the look of a dress.

Interesting that strappy dresses are allowed. At the Christian college I attended, we couldn't wear strappy dresses, but skirts/dresses were allowed to be one hand width above the knee when standing.

OK, my brain is working now ... here's some sites to check out. Great time to pick things up on clearance! These are all great sites to score longer dresses.

http://www.spiegel.com
http://www.newportnews.com
http://www.chadwicks.com
http://www.jcp.com

I don't know what's "cheap" but I'll take a stab at under $100. Also, keep in mind that unless you are 6 feet tall like the models, the dresses listed will fit you longer. Just measure what length you need and check for those measurements online. You can also get a cami bra or a cami if the top is too low cut.
http://shop.nordstrom.com/S/30.....54&P=2
http://shop.nordstrom.com/S/30.....54&P=5
http://shop.nordstrom.com/S/30.....54&P=5
(this would be lovely for evening)
http://shop.nordstrom.com/S/29.....54&P=6
http://shop.nordstrom.com/S/30.....54&P=7
http://shop.nordstrom.com/S/29.....4&P=11
http://shop.nordstrom.com/S/30.....4&P=13
http://shop.nordstrom.com/S/30.....4&P=14
http://shop.nordstrom.com/S/30.....4&P=15
http://shop.nordstrom.com/S/30.....4&P=15
http://shop.nordstrom.com/S/30.....4&P=16
http://shop.nordstrom.com/S/30.....4&P=20
http://shop.nordstrom.com/S/30.....4&P=20

How tall are you? Could you get away with buying a "tall" dress, even though you aren't? THat might give you the extra length. If you are very tall (I'm 6') Boden seems to have the longest talls that I've seen online, often 2-3 inches longer than the tall dresses posted by Gap and Banana (my other go to stores for tall clothing).
Hope that helps...

try TJ Maxx, it's hit or miss but a wide variety. I found two dresses there recently that fit your guidelines. Good luck!

Shabby Apple makes stylish yet modest dresses. They aren't really "cheap" at around $60-80 each, but I think they are a decent value.

If it is impossible to find the clothes you like online, have you considered looking at vintage or thrift stores, dresses from the forties tended to be seamed below the knee.

It may also be a possibilty to choose a pattern and have a local seamstress make a couple of dresses to your requirements.

I hope you manage to find what you are looking for, at my school the skirt had to hit at exactly three inches below the knee and the length would me measured.

Shade Clothing specifically makes "modest" clothing; I have several of their basic tees and tops for layering. They have a few day dresses that might fit your needs:
http://www.shadeclothing.com/P.....s/Overview

Shabby Apple is another "modest" clothing company. They have an interesting convertible dress:
http://www.shabbyapple.com/p-1.....erbal.aspx
http://www.shabbyapple.com/t-apparel.aspx

Good luck, and let us know what you find!