Speech: Writing With (AP) Style

Is it an eighth-grader or an 8th-grader or an eighth grader?

It's the middle one. AP Style, under grade, grader:

Hyphenate in combining forms: a fourth-grade pupil, a 12th-grade student, first-grader, 10th-grader.

Note there is no exception made from the general AP numbers rule of spell out numbers under 10, and use digits for 10 and above. Also note that the examples given appear to adhere to that rule.

Now, in first reference is it PTA or Parent-Teacher Association? Did anybody look under PTA?

Finally, if the number is at the start of a sentence, do you spell it out regardless? You tell me. We've been over this one time and time again.

Please review AP Style under grade, grader and numerals.

Is it Detroit, Michigan, or just Detroit?

It's the latter. Under "Detroit":

The city in Michigan stands alone in datelines.

And under "cities and towns":

See datelines for guidelines on when they should be followed by a state or country name.

There ya go.