American: "math".
British: "maths" <--- from mathematics
"first class" means something to do with quality, so
"First period is ..."
or
"The first period is ..."
"The first class, today, is ...."
"First class, today, is ..."
but "First class is " - sounds a little strange. Not impossible with context, but a little strange to my ear.
.
Try the salad. It's first class.