"I can't offer an offical document either, that the classical language category was introduced by a constitutional decree, but there are many notifications, that are not published on the internet, like the official notification, that Tamil was declared ""Classical language"". However, there are newspaper reports, which confirm, that the classical language category was introduced by constitutional decree. So this is enough to prove it:

http://www.deccanherald.com/CONTENT/Aug142008/editpage2008081384442.asp

Also i found a conversation of the Indian parliament, which illustrates the importance of the classical language status for India, where Mr. Shivraj V. Patil compares the status of the scheduled languages to the status of the classical languages:

SHRI JANARDHANA POOJARY: Sir, the demand for declaring Kannada as a classical language is pending before the Government since 2004. The same answer has been given earlier also. My submission before the House is that the Kannada text and recorded history belongs to 252 BC. The first Kannada literary work Kavirajamarga was created in 850 AD and it refers to some writings earlier than that. The 12th Centaury Kannada Bhakti Movement in the form of Vachana or oral expression is unparallel. In contemporary kirtana which subsequently became Karnataka Sangeet is an enduring contribution to Indian music. Sir, the epigraphical material in Kannada, numbering over 25,000 inscriptions in stone and copper, are rich in style and contents, which are not found even in Tamil inscription. The UNESCO records Kannada amongst the prime scripts of the world. Sir, Kannada is among the first living languages of the world in the history of continuous writing system spanning three millennium like Hebrew, Chinese and Tamil.

SHRI JANARDHANA POOJARY (CONTD): Sir, the recognition was given to Tamil. We do not grudge it. Now, Sir, when the world recognises Kannada, in India...

MR. CHAIRMAN: Please, put your question.

SHRI JANARDHANA POOJARY: ...step-motherly treatment is being given to Karnataka. Agitations are going on in Karnataka. About three days back, about one lakh people gathered. Everyday, it is going on. Has the Government recognised this? Is the Government going to take any action on this, and by what time?

SHRI SHIVRAJ V. PATIL: Sir, Kannada is a very rich language, ancient language. The matter is before the Committee of Linguistic Experts
constituted by the Ministry of Culture. This matter is now with the Ministry of Culture. They are looking into the matter. There are demands made by one other section of the society with respect to their language. Now, these demands are being looked into by the Expert Committee, and after the Expert Committee comes to any conclusion, the Ministry of Culture will take a decision in the matter.

SHRI JANARDHANA POOJARY: Sir, in reply to Unstarred Question No. 1381, it was stated, ""The Ministry of Culture had received presentation from various quarters for declaring Kannada as classical language. A Committee of Linguistic Experts has been constituted to examine the claims of any language classical language status. This Committee will advise the Government in the matter."" This answer was given on 6.3.2006. Subsequently, to me also, the Minister of Culture has replied saying that it has been referred. Now, the question before the House is as to how long it will go on like this. There is a limit to the patience of the people also. They are losing their patience. You may not believe, but they are condemning and criticising the Parliamentarians and saying that Karnataka M.P.'s are dormant; they are inactive.

MR. CHAIRMAN: Please, ask the question. Don't make a statement.

SHRI JANARDHANA POOJARY: How long will the Government take a decision? After being referred to the Expert Committee, there has been a lapse of one year, what action is going to be taken by the Government?

MR. CHAIRMAN: Mr. Poojary, this is a supplementary question. Please, ask the question. Don't make a statement.

SHRI SHIVRAJ V. PATIL: As a matter of fact, this question should have been sent to the Ministry of Culture. But, because, the notice was given to us, and the notice was given at the last moment, the question was accepted and we are replying to it. As a matter of fact, it is only the Ministry of Culture which is in a position to take a decision with respect to this, and I would like to say that the language should unit; language should not divide. Many languages in our country are rich languages and they should be a source of our unity rather than division. So, it would be necessary for all of us to tell our brothers and sisters in the States that on the basis of language, division should not be created. Because the languages are rich; because the languages are ancient; because...(Interruptions).

SHRI JANARDH