Amended on 30 March 2017: Luckily, this tearjerker of an urban myth is properly debunked here.
I very much hope that this story is a hoax, I tried to locate the source on the Internet, but failed.
If it is not a hoax, then it is a huge breach of profession norms- made in a hurry and under stress, but still a breach. One should not put children in the situation of choice almost impossible for them -teachers should remember that. Actually, it is not a good idea to force moral choice on people. In most cases, it is immoral to force moral choice on others.
Added on 30 Dec 2016: As a colleague poindet out, it is likely to be a hoax. Great relief. However, the authencity of the viral twit is doubted because it contradicts protocols for management of blood trnsfusions. 40 year ago in another country, direct blood transfusions were frequently used.