I saw a question which required me to subnet for 400 host. I used a class B IP address but I wanted to use a Class C address and borrow some of the network bits and change them into host, N.N.N.X to N.N.X.X I wanted to know if this is okay, because now the subnet mask looks more like a class B subnet mask.
Classful networking is not used anymore. You can use whaever mask you need. Just be sure that RFC 1918 specifies private IPv4 addresses and those shouldnt appear on the Internet ( They will be probably dropped by first firewall )
Anyway, if the question fully specifies that you HAVE TO consider classful addressing which is obsolate, you can not modify the mask. Thats why it was so wasteful and it is not used anymore