I may possibly be getting an order for a graduation cake to feed around 100 people. I am just starting out and need to get a 1/2 sheet pan anyway.
For those who do sheet cakes...do you put 2 halves together to make a full sheet cake (and just ice over them). Do you layer and fill them or just leave them "just cake?" Does that make sense? Ha!
For a non-filled 1/2 sheet...how many does that feed (reasonably...I'm thinking everyone would probably want a bit more than a 2x2 square...lol). How about a filled 1/2 sheet?
I'm thinking that for 100 people, they would probably need 3 half sheet cakes. But then again, I've only done just smaller bday party cakes and such.
ETA: How many cake mixes go into a 1/2 sheet cake? I'm guessing 2??
I haven't done this but I looked up a similar question under the forums.
http://www.cakecentral.com/cake-decorating-ftopict-631601.html
Hope this helps until someone with more experience comes along.
One 1/2 sheet serves 48 2"x2" peices. So a full sheet is just two halves put together and iced over so you will end up with 96 pieces. I think that 1 Full and 1 half will work great if you think people are going to eat more or bigger pieces. You can put filling in the middle, although I would slide a piece of cardboard underneath to move the top layer or it will break on you. I would also make sure the board underneath is strong or the weight of the cake will make it bend. I usually double or triple the cardboard. I always think 1/2 sheets are easier to deal with. Hope this helps. 
I am doing a grad cake in a couple weeks for 50 people and I am putting two 11x15's together to get almost a 1/2 sheet size. They want to flavors so I had to do it this way. I am planning to fill it with buttercream. I figure it should do about 50 people for a 1/2 sheet. I may do it a little bigger like two 12x18's together to make sure.
My guess for 100 people would be 2 - 1/2 sheet cakes but you can look online for serving size charts to help with that. I am planning on two mixes for a double layer 1/2 sheet total size...hopefully one mix of each flavor since I doctor them to add more batter to it.
The filled double cakes can be cut smaller than a single cake too so you would get more servings out of a filled cake. Something to think about 
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Hope this helps, I am still trying to figure it all out myself, lol, so I hope more people give their advice so I can see what they think.
Here's a very similar post w/helpful answers:
(Includes link to Everything you need to know to make, decorate and assemble tiered/stacked/layered/sheet cakes.)
http://www.cakecentral.com/cake-decorating-ftopict-628706-.html
A full recipe of the WASC cake (included in link above) made with DH white cake mixes yields a tad over 14 cups of batter, so it's great for large or multiple pans. (Per Wilton, a 12x18x2 pan requires 14 cups of batter.)
HTH
Here's what I do and have done:
* 12x18 = half sheet cake ( and for the record, I REFUSE to use the terms "half" and "full").
* 12x18, single layer, not filled, iced in buttercream is a "sheet cake".
* 12x18 single layer serves 54 (2x2x2)
* Two 12x18's side by side (now you have 18x24 ... full sheet) = 108 servings (2x2x2)
* Two 12x18's stacked on top of each other with filling (a 12x18, 2-layer cake) serves 108 (1x2x4).
* 12x18 pan uses 3 cake mixes. (I use Betty Crocker. I understand Pillsbury doesn't yield as much batter.)
A 2x2x2 piece of cake, and a 1x2x4 piece of cake is bigger than you think. What I do is tell people they will get about 100 pieces from an 18x24 "....if you cut it 2x2x2 ...if you plan to cut it bigger, you should probably plan to order more cake."
Is this for a grad open house or a grad ceremony? If it's an open house, remember that folks will be going from house to house and as my sister pointed out, "People get caked out", which is why she ordered cookies for her son's graduation instead of cake. So unless this is the only grad party that day in the whole town, people will probably just want a "polite" piece of cake rather than a "brick" piece of cake. But it's your customer's call.
looks like my cake will be wayyyyy too big...maybe I'll do a 12x 18 to feed 50? So you could do 2 of them to feed 100?
I do cakes for the Governor's office pretty often. When they order cake for 100, I make two 12x18's. Works fine every time. (ooooh! a rhyme! ). Believe me, this is one order that I dont' want to run short on!
Two 11x15's will serve 70 (35 each) so yours wouldn't have been too far off.
To figure servings on a square or rectangle cake, it's easy to do the math. When cut in 2" x 2" squares, you'd cut the 12" side of the cake into 6 rows (12" divided by 2" pieces = 6 rows) and you'd cut the 18" side into 9 columns (18 divided by 2" = 9). 9x6 = 54 servings.
Thanks Indydebi...I always feel better when you chime in . I think I may stay with the bigger cake afterall so they can cut good size pieces and have left overs...they have alot of kids, lol!
Twingirls05- I hope all of this is helping you out a little
I do cakes for the Governor's office pretty often. When they order cake for 100, I make two 12x18's. Works fine every time. (ooooh! a rhyme! ). Believe me, this is one order that I dont' want to run short on!
i agree with indydebi,
sheet cake were all i did for the longest time. 1/2 sheets will not feed 100 people. i would do 2 of them for that many people. 
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