Is 12th class tuition necessary?

Its not necessary, it depends on you. If you have some difficulty in understanding some concept, first try to check for online solution. In today's world, information is free. If still you do not understand, then ask someone for help, to your school teacher or private tutor. Make your own calendar and fix the time for each subject. Choose the subjects which you want to pursue in future and accordingly put more focus on those subjects. Finally, solve more and more question papers online and get it checked by someone. Hope this helps.

I am a teacher of Maths and Physics for Class 11 & 12.

Yes tutions are necessary these days for class 12 students because they need to score good marks for procuring the admissions in in reputed colleges as per their dream streams.

Tuitions actually save their time in understanding the complex concepts with the help of the tutor in Fixed time so that 8A get ample time to practice and master the skin with proper understanding of the subject.

Class 12th is a very important step in your life.there are many colleges and compeititve exams who give great weigtage to the marks obtained in CLass 12th Examination.
Tution can help you with completing the chapter in the given time,with a better understanding of concepts.they will be taking several mock test based on previous year exams which will make you ready to face the actual board examination.tution help the students get that individual attention that they miss in school and ask doubts whenever needed.
But if your school classes are doing good,you are able to understand the concept well and able to solve the questions well,then its good.with this if you are a punctual and detremined student towards you studies and know how to startegize your timing to complete the course and take self mock test,then "Not taking tutions" can be considered as a good option.
but being a good with studies and not being able to strategize well on how to complete the syllabus course,is also hinting towards tution only.The right tution can make your journey easy and simplistic. 
think very wisely before coming to a decision.