Possible 455?

In my school auto shop there's this Pontiac engine- and under the left side of the block it has 455 imprinted in it. Does this mean its a 455?

Yes. The date code is on the back, next to the distrobutor. It will be an alpha-muneric code. The first digit is a letter, the last three are numbers. The Letter indicates the month of casting. The first two numbers are the day of the month, and the last number is the last digit of the year. Example: D224. April 22, 1974.
There is also a two letter code stamped (not cast in) on the passenger side front of the block. This indicates the specific application. "YD", is a common one. YC is another. Usually, "Y" indicates automatic transmission, "W" indicates manual.
By getting the date code and application code, you can narrow it down to what type car it was originally installed in.
Are there heads? The casting number of the head will also give you a clue to the era. The casting number is located on the outside of the center exhaust ports. Example: 4X (a common 455 head).
These engines have tremendous power potential, and are among the best street performance engines ever offered by anyone. Even if there are no heads, grab it! Heads are easy to come by, and 455 heads are usually not desired (chamber too large for good compression). 350 (PONTIAC) and 400 heads are more desirable.