Technically, apparently, your teacher is correct.
BIPM and NIST
In the official brochure from the Bureau international des poids et mesures (BIPM, the keepers of SI units) in §5.1 Unit symbols we find:
It is not permissible to use abbreviations for unit symbols or unit names, such as sec
(for either s or second), sq. mm (for either mm2
or square millimetre), cc (for either
cm3
or cubic centimetre), or mps (for either m/s or metre per second). The use of the
correct symbols for SI units, and for units in general, as listed in earlier chapters of
this Brochure, is mandatory. In this way ambiguities and misunderstandings in the
values of quantities are avoided.
In NIST's official SI guide, §6.1.8 we find (thanks to Emilio):
6.1.8 Unacceptability of abbreviations for units
Because acceptable units generally have internationally recognized symbols and names, it is not permissible to use abbreviations for their unit symbols or names, such as sec (for either s or second), sq. mm (for either mm2 or square millimeter), cc (for either cm3 or cubic centimeter), mins (for either min or minutes), hrs (for either h or hours), lit (for either L or liter), amps (for either A or amperes), AMU (for either u or unified atomic mass unit), or mps (for either m/s or meter per second). Although the values of quantities are normally expressed using symbols for numbers and symbols for units (see Sec. 7.6), if for some reason the name of a unit is more appropriate than the unit symbol (see Sec. 7.6, note 3), the name of the unit should be spelled out in full.
Both sources agree that when printed, the unit's name should appear as the lowercase, non-italic: "ampere" and its symbol should be the uppercase roman script letter: $\mathrm{A}$.
When spoken, people regularly use "amp" as short for "ampere" (which never causes confusion), though it would appear, according to official recommendations, in printed material the full name should always be used.
IEEE
For engineers (specifically the IEEE) the policy seems more lax, though in an amusing way.
Specifically in IEEE Std. 260-1 [doi] we find in footnote 1:
The unit names ampere and second
have sometimes been abbreviated amp and sec, respectively, but this usage is now deprecated. The standard unit symbols for ampere
and second are A and s, respectively
But then in 2014 IEEE Standard's Style Manual [pdf], regarding footnotes in §16.2 Footnotes we find:
Mandatory requirements shall not be included in text footnotes because these footnotes are not
officially part of the standard.
so it would appear IEEE members are unbound from the deprecation mentioned.