sending message using android app and web on php, javascript

can anyone give me an idea on what to do. we are doing student portal system. we will do an app for phone and a website. our system has messaging wherein the teacher can send message to students. could you give me an idea on what to use in the messaging feature. im thinking of database but im afraid that it might slow our system. what should we do. the message must link to the web and anroid app. thanks