If an average student gets into top B-schools like XLRI, JBIMS, IIFT, and FMS, how can he/she standout in the summer and final placements?

If you are an average student then your previous experience, CV, and performance in GD/PI would matter the most in final placements.

Experience: If you have any experience then companies are likely to pick them for similar roles during summers and final placements.

CV: Focus on the matters the most, CV construction and articulation. There are plenty of sites on the internet to help. A good CV will help you to get many more interviews than other batchmates. One should put a lot of effort into that.

GD/PI: Preparation for Group Discussion and the Personal Interview should be a mixture of personal questions, current affairs, academic questions, industry or sector, company in that order. 

• Employers don't expect deep insights but expect high levels of awareness from you and cultural fit is a must. 
• If you are highly ambitious then the foreign banks will prefer you. 
• If you are hardworking and can demonstrate that, the local banks will prefer you. 
• If you are not afraid of rolling up your sleeves and happy working with small shops, rural areas, the FMCG may select you.

You should be able to articulate your interview to be able to demonstrate these attributes. The interviewers are usually experienced enough to see through the lies of the students.