I need to make a large sheet cake for my son's school. I have found tons of info on how much batter and cooking times but I can't find anything about layers and more importantly how do i get it out of the pan in one piece? If it is just one layer I could just put a cake board over the pan and flip but what do you do for layers on a cake that big? Also which size is truly a full sheet 12x18 or 14x22?

A full sheet cake is 18x24
I flip the cake out the same way I do my smaller cakes - onto a cooling rack - if you don't have one that large, flip it onto parchment on a cake board

Neither. A commercial full sheet is 18x26. A 1/2 sheet is 12-7/8'' x 17-3/4. But a "sheet cake" is the most generic term used in baking, it's whatever size the bakery wants to make. Even though everyone technically buys 1/2 sheet cakes, everyone asks for a "sheet cake".

The easiest way to handle a 1/2 sheet to to freeze your layers, and handle them partially frozen. Depending on how delicate your cake is, some people can flip and toss their 1/2 sheets around (Cake Boss), or some people slide off another board. Lots of people don;t even fill theirs, it's juts one layer of cake with frosting on it.

How many people does it need to serve? Unless you need to serve hundreds of people then single layer sheet cakes work best. It is also easier to serve and takes up less room on a plate. 2x2 is the normal serving size for single layer sheet cakes.

A single layer 12x18 serves 50 people it is also called a 1/2 sheet. Unless you have a large oven you would need to make 2 of this size and put them side by side to have a full sheet cake. You can also stack them on top each other. A full sheet serves 100.

When turning the cake out place a large rectangular cooling rack over the cake pan and flip pan, cake and rack over. then lift the pan off.

I've not baked a sheet cake as large only a 9" x 13" , I only split this into two layers and used a large cake lifter to gently lift between the layers to wedge a large flat baking sheet in between to lift the top layer off so that I could layer the buttercream on the bottom layer and then replace the top layer by gently sliding it off the baking sheet back onto the bottom layer. I hope this helps

I would just make 4 9x13's and jam them together on a big plywood cake board. Do each one individually and it is easier. Do you even have a pan or oven that big?

In most cases 'sheet cakes' are only 1 layer (2"). It can be split, filled and iced or not. Other people make 2 layers - actually baking 2 cakes and stacking them w/filling. It depends on how many servings you need.
.........just make 4 9x13's and jam them together on a big plywood cake board........
Problem w/this is how to deliver it. One MUST make sure it will fit through all doors or hallways!
..........When turning the cake out place a large rectangular cooling rack over the cake pan and flip pan, cake and rack over. then lift the pan off..........
Definitely a great way to handle lg rectangle cakes.
Someone suggested fzing the cake for easier handling but one must get it out of the pan 1st Once it's out you can chill/fz it but most of us find it;s not necessary. Don't be afraid to handle them. Most cakes are not *that* tender and handle well.

Thanks for the reply's. It is for my son's school and they want one big enough to feed 100 people. Thank you so much for reminding me about doorways. I would have completely forgotten about that.