Some of our clients are currently experiencing problems when sending large batches of SMS problems via SMSWeb. The problem appears to be when messages are sent to large number of recipients (>1000) and when the message text is also long.
We are working with SMSWeb to try and resolve this issue, but for the moment please check that SMSes you send to large groups are being sent successfully.
If you find that a message to a large number of recipients does not get sent you can try breaking up the recipients into smaller groups and sending to those groups e.g. instead of sending to the whole school at once send to phases or grades at once.
UPDATE: Note that this issue appears to be resolved now.