One of my cousins graduated from IIM Indore and according to him, it is not really a disadvantage.

• Extra-curricular serves the purpose of letting you speak about something other than your regular academics. It shows the panelists that you not just focused on your academics but you were also active in other activities.
• Now, not having extracurricular achievements does not mean you won’t be able to talk about anything. Be thorough in your subjects from 12th grade. Make sure you know the relevant concepts and can apply them in real-life situations. 
• Also, know about your school, its history, and basic trivia around it. Know about your home state - famous personalities belonging to your state, dance forms, festivals, and the likes.
• Talk about your hobbies and about how you've developed them over the years. 

Lastly, be aware of current affairs. Read newspapers regularly and make sure you are able to form opinions and support them with logical facts.