@Nancy
If you enter the Schengen Area in particular as a tourist you are not entitled to undertake work of any kind be it voluntary, in exchange for accommodation or other services and of course the paid kind. Failure to comply with these rules will result in you at a minimum being deported and more likely being fined and being excluded from the Area for a fixed or indefinite period.
It is also worth pointing out that we have civil law in most of Europe rather than common law as in the US and the British Isles, which makes the application of the law very black and white. There is no obligation on the authorities to consider your circumstance or excuses when applying the laws. Attending a school or similar institution on a regular basis for the purpose of conversing with the students is not a tourist activity. That is how they will see it.
Of course organizations are happy to have you volunteer with them, because if it all goes wrong you are the one taking all the risks not them. For them it is simply a case of 'Oops sorry about that', while you sit in a detention center waiting to be deported.
If you intend to do anything other than be a tourist the best advice is to seek written confirmation from the relevant embassy that you do not need a work permit or other documentation before you do so.