The phrase "are several classes of" is correct and usable in written English.
It can be used when discussing different categories or types within a particular subject or field.
Example: "In the study of biology, there are several classes of organisms, including mammals, reptiles, and amphibians."
Alternatives: "there exist multiple categories of" or "there are various types of".
There are several classes of bike rider.
There are several classes of shamans, with different abilities.
There are several classes of oral drugs used to control blood glucose levels, including sulfonylureas, biguanides, and thiazolidinediones.
There are several classes of data structures that are very useful to avoid the processing of empty cells.
There are several classes of Center grants: Core Center (P30), Specialized Center (P50), Comprehensive Center (P60), and Planning Center (P20).
Reviewed here are several classes of small molecule fluorescent sensors designed to detect several molecular participants of oxidative stress.
Politico's Maggie Haberman reports there will be several classes of attendee at the retreat.
Prominent among those induced by both fungi, as noted above, were several classes of antimicrobial peptide genes, as well as the 4 insulin-like genes, ins-7, ins-11, ins-23 and ins-36.
To be able to accurately characterize and quantify the behavior of what appeared to be several classes of osk transcripts, we used an advanced mathematical algorithm [30], [31], [32], [33], [34] integrated into an image analysis software (QUIA) developed specifically for the purpose of the detection and tracking of such particles in vivo.
These individuals came to a consensus that there were several classes of resources that did not have any accepted risk-of-bias assessment tools or instruments (e.g., textbooks, narrative reviews, learning guides, expert opinions).
There are several classes in the school of rock in 2010.
