I’m a high school student. I wish to make a career as a scientist in Environmental Science. Please tell me about the institutions and salary in this field.

Environmental Scientists are mainly concerned with research on environmental pollution and developing ways to control it. You’ll work on everything — air pollution control, greenhouse gases, management of toxic waste and biological waste, chemical waste and various other types of waste ozone depletion, side effects of pollution on human beings, animals and plants. Water quality management, land conservation, soil conservation, etc. will also be your areas of work.
To become a scientist, you should go for M.Sc. (Environment Science) followed by a Ph.D. Eligibility for M.Sc. is graduation in science subjects.
Some of the universities which offer this course are:

I have recently completed my 10+2 in Science (PCMB) and want to go for pilot training or technical studies. But, because of the high fees, I have applied for a B.Sc. Hons in Mathematics at St Stephen's College, Delhi. What are the career options after finishing this course?

If you are really keen on becoming a pilot, there is another route, join the Indian Air Force and become a flying officer. There you won't pay such a hefty sum of money for training. After finishing your graduation, you can enter the flying branch through the Air Force Academy, which is one of the most prestigious training establishments of the Indian Air Force. Here you'll be trained and prepared for your responsibilities as a pilot. Selection is through the Combined Defence Services Examination (CDSE). Upon successful completion of the training, cadets are absorbed as permanent commissioned officers. For further details visit: http://careerairforce.nic.in/
Otherwise, as a graduate in Mathematics, you can find the financial world, environmental modelling, oceanography or meteorology or develop careers in computing and information technologies. You can also apply for the post of Probationary Officer in various banks.
There are varied opportunities for those with a master’s degree. They may work in research and development departments and be involved working on the problems of another field by using their skills in modelling and analyzing. Post-graduates in Statistics/Mathematical Statistics/Applied Statistics can appear for the Indian Statistical Service (ISS) examination, conducted by UPSC.

I’m 15 years of age studying in class 10. My parents want me to join IIT but I want to be an IAS officer. Maybe I should opt for IIT because it carries a lot of money. Due to all this confusion, my academic performance has gone down from 89% to 75%. I feel ashamed to be with friends who still get a good percentage. What should I do to stop this kind of feeling?

Forget about money, first focus on your capabilities and interest. What do you really want? Engineering is a technical field, and engineers basically design, create, and implement processes, machinery and systems. But after becoming an IAS officer you will be involved in administrative type of work. Even if you are not clear about your preference at this point of time, focus your energies on studies. Both the fields require thorough studies. Once you get yourself involved in studies, your marks and hence confidence will improve.
For entering into an IIT, you need to appear in IITJEE after 10+2 with Physics, Mathematics and Chemistry. The competition requires extensive study of 10+2 syllabus. Indian Administrative Service (IAS) is considered to be the most powerful, glamorous and coveted of all Indian services. It attracts some of the best brains in the country. It covers the entire government bureaucratic machinery from the remotest rural areas to the Central Cabinet.
The basic eligibility for appearing is graduation in any stream, including engineering. This is truly an intense competitive examination and requires at least one year of dedicated preparation. Though the salaries in the civil service are not high compared to the private sector, and transfers are a common feature but it is the perks, status and high level of recognition that makes it very popular.

I am a 12th science student with maths group. I want to know which career in this area and current scenario is the best as the IT field has broken down. Which other areas offer good career prospects?

During the boom years for the software industry, all types of people entered the market, many of whom made huge money. The story was good for a while, but the bubble was going to burst one day. Today, there is less demand for software professionals and thus preferences for IT-related courses are decreasing.
But IT industry will soon stabilise and become like any other economic sector or industry in the country, with the same opportunities. Only difference will be that instead of waiting at the doors of educational institutions to lap up people, IT companies will now choose ’good’ graduates. A mere certificate from some hole-in-the-wall institute will not work.
So if you’re really interested in IT field, you do not need to worry. If you love your field, are good at what you do, have the necessary education/training/certifications, and can master job-search skills, then you should be able to find a job.
As far as other fields are concerned, education, telecom services, life sciences, bio-technology, nano-technology, finance and banking, retail financing, actuarial, media & entertainment and consulting services also offer good career prospects.