Is it normal to be mentally incapable of passing high school?

Since the vast majority of people do manage to finish high school, not being able to do so isn't "normal" in that sense.

However, many who do manage to get through to graduation find the experience anything from stressful to totally hellish for a variety of reasons.

Some people just aren't intelligent enough to deal with the academic side, and others find the social aspects extremely difficult to deal with.

depends what year your in. from my experience, it gets better after year 9.

Not everyone is cut out for academia. You might be a more visual, aural or physical person and school doesn't tap into that. Some people who fail miserably at school are excellent artists, dancers or muscians, or they find physical work like construction or landscaping more up their alley. Even yoga instructors! There's always hope!

You show up enough days and you at least pass.

Mentally incapable of finishing high school, in what way?

Yes normal.