If you are planning to study abroad especially in the US or in the UK, health insurance is necessary simply because the cost of healthcare in this countries is very high.
As with any product in free market economies, students are faced with many choices. However most universities in the US have a tie-up with local companies to provide insurance to their students. In such cases the choice is relatively easy. If there are no such arrangements, then you can look at student insurance policies offered by companies in India.
Students will need to check applicable costs as there is a difference of almost Rs 4,000 on an annual basis in the cover taken in India and that taken abroad.
The bottom line is that there must be adequate cover for at least three years as this is the minimal time for most courses abroad.