Hi aspirant,

It is absolutely a rumour and has no relevance in actual life. Actually the pre Board examinations are conducted by schools to help student in preparation for the board examination and so that they pay attention to the  examination seriously these rumours are created. Choosing a stream is completely dependent on the candidates choice and only the performance in the board examination is taken into consideration while following the same. However I would request you to give the pre Board examinations with full concentration as it would be certainly helpful in preparation for board examination.

Hope it helps!