I read this sentence from the internet. I think it there is

a grammar error:

Both "are subject" to government-controlled assessment and monitoring, but parents foot the major part of the bill in the Independent schools.

Shouldn't this be "are subjected" ??

No it is fine as it is.

Subject to (Transitive verb)

Cause to have an unpleasant experience (which I’m sure this could be, but it is not the meaning wanted here.)

e.g. They were subjected to a vigorous scrubbing.

Subject to (adjective)

Likely to be affected by a particular thing

e.g. The area was subject to flooding.

"subject" in "be subject to" is an adj. meaning
here "under the authority of" or "controlled by"