• 教室きょうしつ — classroom
• 室内しつない — indoor / inside the room
• 和室わしつ — Japanese-style room
• 洋室ようしつ — Western-style room
• 寝室しんしつ — bedroom
• 客室きゃくしつ — guest room

More examples ▼

• 浴室よくしつ — bathroom
• 温室おんしつ — greenhouse
• 待合室まちあいしつ — waiting room
• 会議室かいぎしつ — meeting room / conference room
• 美容室びようしつ — beauty salon / hair salon
• 皇室こうしつ — Imperial Family
• 入室にゅうしつ — entering a room
• 退室たいしつ — leaving a room

室 combines 宀 (roof) with 至 (to reach). Originally, it meant a protected inner space in a house. Today it’s used for many kinds of rooms and indoor locations.