יֵשׁ לִי לָלֶכֶת לְבֵית סֵפֶר מָחָר. (Yesh li lalekhet le-Beit Sefer machar.)
* יֵשׁ לִי (Yesh li) - I have
* לָלֶכֶת (lalekhet) - to go
* לְבית סֵפֶר (le-Beit Sefer) - to school (literally "to house of book")
* מָחָר (Machar) - tomorrow
You could also say: אני צריך ללכת לבית ספר מחר (Ani tzarich lalekhet leBeit Sefar Machar) which translates to "I need to go to school tomorrow". This is a slightly more emphatic way of saying it.