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In terms of your question about this particular tattoo, most schools will find this inappropriate for various reasons.
A tattoo of any kind can be fine but covered up is that is expected often from a school board and/or principal. The hour 4:20 can be seen as the meaning for weed and even more so can be brought up as a problem.
Since I would think the tattoo is not going to be removed, suggestion is to purchase longer trousers so that the tattoo is covered and no cause for concern will be seen.