The sentence "我想提醒你我明天不会上课" (Wǒ xiǎng tíxǐng nǐ wǒ míngtiān bù huì shàngkè) is grammatically correct, but the use of the word "提醒" (tíxǐng) in this context may come across as slightly direct or abrupt. To convey your message politely, you can consider using a more polite expression. For example, you could say:--"请允许我告知您，明天我无法参加上课" (Qǐng yǔnxǔ wǒ gàozhī nín, míngtiān wǒ wúfǎ cānjiā shàngkè).--or–"很抱歉通知您，我明天不能上课" (Hěn bàoqiàn tōngzhī nín, wǒ míngtiān bùnéng shàngkè).--Both of these alternatives maintain a polite tone while conveying your message about missing class tomorrow.