It seems to me that I hear this particular grammar error more frequently now than in the past. I don’t know why, and it doesn’t really matter; I’m just here to clear things up.
Schools will open at 9:30 a.m. this morning. The show will start at 10:30 p.m. this evening.
A.M. means ante meridiem, which is Latin for before noon (midday); therefore, saying Schools will open at 9:30 a.m. this morning is redundant since it means Schools will open at 9:30 before noon this morning. Is there any other 9:30 before noon than the one that occurs in the morning? Is there a 9:30 before noon in the evening or afternoon?
You could say The school will open at 9:30 a.m. or The school will open at 9:30 this morning or The school will open at 9:30 a.m. today.
The same could be said for The show will start at 10:30 p.m. tonight. P.M. means post meridiem, which is Latin for after midday, so this sentence is a redundancy. It says The show will start at 10:30 after midday this evening. Is there any other 10:30 after the hour of noon (midday) than the one that occurs at night? Is there a 10:30 after the hour of noon in the morning?
By the way, the abbreviation a.m. has several meanings, some of which are distinguished by whether it is capitalized. For ante meridiem, the correct spelling is with lowercase letters, a.m., since it is an abbreviation for ante meridiem, not Ante Meridiem. You also need the periods to distinguish between meanings (for example, A.M., Am, am). The same is true for p.m.