You could ask them to confirm that they received your text, rather than repeating it (more or less) verbatim:
hi just wondering if you got my text re Monday 31st
That way it avoids blaming them for ignoring the text, as it asks indirectly about 'technical issues'.
Also, send it at a time that is appropriate - if you send it in the middle of a lesson, they may read it but forget to reply, because they are concentrating on something else.