What you are typing is a DOS command to change to the "8051" directory. You need to provide the full path to the "8051" directory after the cd, for example;
CD school8051
If the "8051" directory does not exist then you will need to provide the actual directory name where the assembler program resides.
Also, is this a legitimate copy or have you approriated a licensed copy from the school? There are plenty of free assemblers available in demo IDE packages such as Raisonance or Keil. Metalink also provides a free assembler. All of these provide comprehensive manuals for the installation and use of the program. Check the LINKS button at the left.

Hal