It is usually a white or yellowish powder, which is snorted, but can also be obtained in pills and capsules. It is marketed as plant food.
Police made three arrests following the death of the two teenagers.
NAHT general secretary Mick Brookes told the BBC the use of the drug had become more widespread.
He said: "This drug clearly has the same inherent dangers as any Class A drug and I think serious consideration should be given to banning it.
"The problem with that is that you then criminalise the people who take it, so we need to think very carefully about what we do, but act with some speed."
Some heads say the drug should be made illegal immediately - even if it risks some children getting a criminal record.
Schools have become increasingly worried that because the drug is legal, children as young as 12 are trying it.
BBC science correspondent Tom Feilden said the Advisory Council on the Misuse of Drugs (ACMD) was asked by the Home Office last year to examine the use of legal highs such as mephedrone.
A sub-committee was set up to do just that, and was due to report back this month, our correspondent added.
Is it too easy to buy a 'legal high'? (first broadcast Dec 2009)
However, the work of the committee on legal highs stopped following the dismissal of its chief drugs adviser Professor David Nutt last October. He was sacked after publicly disagreeing with the government's decision to reclassify cannabis as a Class B drug and not to downgrade ecstasy.
Five ACMD members then resigned in the row that followed Prof Nutt's departure.
Websites selling the drug have told their customers it is a case of "when" not "if" mephedrone will be banned.
The two teenagers who died were found collapsed at their homes after a night out in Scunthorpe.
Humberside Police said the drug appeared to have contributed to their deaths. They had been out drinking in the hours before their deaths.
Head teacher Mike Stewart: 'We need help'
Two men, aged 26 and 20, and a 17-year-old boy have been arrested in connection with the incident.
Post-mortem examinations are being carried out.
The substance has become popular on the UK club scene in recent months.
Its side effects are said to include psychosis, palpitations, burning throat, nosebleeds and insomnia.
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