Tian Zi Ge (田字格) paper, US Letter
Tian Zi Ge character squares laid out for an 8.5 × 11 inch sheet, so the squares come out at a true 20 mm on North American paper.
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A Chinese format on American paper
Tian Zi Ge is a Chinese and Taiwanese school format, and printed pads of it come in A4 or in Chinese standard sizes. If you are teaching or learning Chinese in the United States or Canada, the paper in the printer is Letter, and an A4 sheet fitted to Letter by the driver silently shrinks every square below 20 mm. This is the same grid built for the page you actually have.
Squares to a page
At 20 mm a Letter sheet holds about nine columns and twelve rows inside normal margins — a little over a hundred characters, which is roughly one homework exercise. If you need a specific count for an assignment, change the spacing rather than squeezing the margins, because character size is usually the thing being assessed and the margins are what keeps the sheet legible.
Which guides to show
Plain Tian Zi Ge has a cross only — one vertical and one horizontal through the centre, dividing the square into four. Adding both diagonals turns it into Mi Zi Ge, which gives eight reference sectors and helps beginners place diagonal strokes. Start with Mi Zi Ge if proportion is the problem, and move back to the plain cross as soon as it is not.
Printing it accurately
This only measures correctly if your printer doesn't rescale the page. In the print dialog set Scale: 100% — not "Fit to page" or "Shrink oversized pages". If you're not sure it worked, print the calibration page, measure the ruler on it with a real ruler, and it will tell you exactly what your printer is doing to the page.